diff --git a/web/package.json b/web/package.json
index 50e10d3b..3cba9e3d 100644
--- a/web/package.json
+++ b/web/package.json
@@ -14,7 +14,7 @@
     "test:css": "stylelint **/*.css **/*.svelte",
     "test:build": "pnpm build && size-limit",
     "build:visual": "NODE_ENV=test storybook build",
-    "build:web": "vite build && tsx ./scripts/clean-vars.ts",
+    "build:web": "vite build && tsx ./scripts/clean-css.ts",
     "format:stylelint": "stylelint --fix **/*.{css,svelte}",
     "clean:dist": "rm -rf dist"
   },
diff --git a/web/postcss/roots-merger.ts b/web/postcss/roots-merger.ts
new file mode 100644
index 00000000..e1a0f350
--- /dev/null
+++ b/web/postcss/roots-merger.ts
@@ -0,0 +1,45 @@
+// PostCSS plugin for ../script/clean-css.ts to merge all rules
+// with :root selector into the single rule in the beginning of the file
+
+import type { ChildNode, Plugin, Rule } from 'postcss'
+
+export const rootsMerger: Plugin = {
+  postcssPlugin: 'roots-merger',
+  prepare() {
+    let rootNodes = new Map<string, ChildNode>()
+    let rulesToRemove: Rule[] = []
+
+    return {
+      OnceExit(root, { Rule }) {
+        rulesToRemove.forEach(rule => rule.remove())
+
+        let rootRule = new Rule({ selector: ':root' })
+
+        rootRule.append(...rootNodes.values())
+        if (rootRule.nodes.length > 0) {
+          root.prepend(rootRule)
+        }
+      },
+      Rule(rule) {
+        if (rule.selector !== ':root') {
+          return
+        }
+
+        if (rule.parent?.type === 'atrule') {
+          return
+        }
+
+        rule.walkDecls(decl => {
+          // remove rule from the map to preserve the rules order
+          if (rootNodes.has(decl.prop)) {
+            rootNodes.delete(decl.prop)
+          }
+
+          rootNodes.set(decl.prop, decl)
+        })
+
+        rulesToRemove.push(rule)
+      }
+    }
+  }
+}
diff --git a/web/postcss/vars-cleaner.ts b/web/postcss/vars-cleaner.ts
index e30bdb9f..3c9315ec 100644
--- a/web/postcss/vars-cleaner.ts
+++ b/web/postcss/vars-cleaner.ts
@@ -1,4 +1,4 @@
-// PostCSS plugin for ../script/clean-vars.ts to remove unused palette colors
+// PostCSS plugin for ../script/clean-css.ts to remove unused palette colors
 // and throw error on unused CSS Custom Properties.
 
 import type { Node, Plugin } from 'postcss'
@@ -16,8 +16,8 @@ function removeWithEmptyParent(node: Node): void {
 let globalUsed = new Set<string>()
 let globalVars = new Set<string>()
 
-export let cleaner: Plugin = {
-  postcssPlugin: 'clean-vars',
+export const varsCleaner: Plugin = {
+  postcssPlugin: 'vars-cleaner',
   prepare() {
     let used = new Set<string>()
     let vars = new Map<string, Node[]>()
@@ -53,7 +53,7 @@ export let cleaner: Plugin = {
   }
 }
 
-export function checkUsed(): string[] {
+export function getVarsCleanerError(): string | undefined {
   let unused = []
   for (let name of globalVars) {
     if (!globalUsed.has(name)) {
@@ -62,5 +62,15 @@ export function checkUsed(): string[] {
       }
     }
   }
-  return unused
+
+  if (unused.length === 0) {
+    return
+  }
+
+  return `Unused CSS variables: ${unused.join(', ')}`
+}
+
+export function resetCleanerGlobals(): void {
+  globalUsed = new Set<string>()
+  globalVars = new Set<string>()
 }
diff --git a/web/scripts/clean-vars.ts b/web/scripts/clean-css.ts
similarity index 69%
rename from web/scripts/clean-vars.ts
rename to web/scripts/clean-css.ts
index 57b3c9ce..d0f0c220 100755
--- a/web/scripts/clean-vars.ts
+++ b/web/scripts/clean-css.ts
@@ -1,4 +1,6 @@
-// Remove unused colors from palette (as CSS Custom Properties)
+// 1. Merge all rules with :root selector into the single rule
+// in the beginning of the file
+// 2. Remove unused colors from palette (as CSS Custom Properties)
 // and throw and error if other CSS Custom Properties are unused.
 
 import { lstat, readdir, readFile, writeFile } from 'node:fs/promises'
@@ -6,7 +8,8 @@ import { extname, join } from 'node:path'
 import pico from 'picocolors'
 import postcss from 'postcss'
 
-import { checkUsed, cleaner } from '../postcss/vars-cleaner.js'
+import { rootsMerger } from '../postcss/roots-merger.js'
+import { getVarsCleanerError, varsCleaner } from '../postcss/vars-cleaner.js'
 
 function printError(message: string | undefined): void {
   process.stderr.write(pico.red(message) + '\n')
@@ -14,16 +17,21 @@ function printError(message: string | undefined): void {
 
 async function processCss(dir: string): Promise<void> {
   let items = await readdir(dir)
+
   await Promise.all(
     items.map(async name => {
       let path = join(dir, name)
       let stat = await lstat(path)
+
       if (stat.isDirectory()) {
         await processCss(path)
       } else if (extname(name) === '.css') {
         let css = await readFile(path)
+
         try {
-          let fixed = await cssCleaner.process(css, { from: path })
+          let fixed = await cssCleaner.process(css, {
+            from: path
+          })
           await writeFile(path, fixed.css)
         } catch (e) {
           if (!(e instanceof Error)) {
@@ -42,12 +50,11 @@ async function processCss(dir: string): Promise<void> {
 
 const ASSETS = join(import.meta.dirname, '..', 'dist', 'assets')
 
-let cssCleaner = postcss([cleaner])
+const cssCleaner = postcss([rootsMerger, varsCleaner])
 
 await processCss(ASSETS)
 
-let unused = checkUsed()
-if (unused.length > 0) {
-  printError(`Unused CSS variables: ${pico.yellow(unused.join(', '))}`)
+if (getVarsCleanerError()) {
+  printError(getVarsCleanerError())
   process.exit(1)
 }
diff --git a/web/test/roots-merger.test.ts b/web/test/roots-merger.test.ts
new file mode 100644
index 00000000..780fa471
--- /dev/null
+++ b/web/test/roots-merger.test.ts
@@ -0,0 +1,71 @@
+import { equal } from 'node:assert'
+import { test } from 'node:test'
+import postcss from 'postcss'
+
+import { rootsMerger } from '../postcss/roots-merger.js'
+
+function run(input: string, output: string): void {
+  let result = postcss([rootsMerger]).process(input, { from: undefined })
+  equal(result.css, output)
+}
+
+test('merges all :roots into one', () => {
+  run(
+    ':root { --color-red: #f00; }' +
+      ':root { --color-blue: #00f; }' +
+      '.some-class { color: tomato }' +
+      ':root { --color-green: #0f0; }',
+    ':root {' +
+      ' --color-red: #f00;' +
+      ' --color-blue: #00f;' +
+      ' --color-green: #0f0 ' +
+      '}\n' +
+      '.some-class { color: tomato }'
+  )
+})
+
+test('merges all :roots into one and respects last defined variable value', () => {
+  run(
+    ':root { --color-red: #f00; }' +
+      ':root { --color-blue: #00f; }' +
+      ':root { --color-blue: blue; }' +
+      '.some-class { color: tomato }' +
+      ':root { --color-green: #0f0; }',
+    ':root {' +
+      ' --color-red: #f00;' +
+      ' --color-blue: blue;' +
+      ' --color-green: #0f0 ' +
+      '}\n' +
+      '.some-class { color: tomato }'
+  )
+})
+
+test('merges all :roots into one except :roots with classes', () => {
+  run(
+    ':root { --color-red: #f00; }' +
+      ':root.blue { --color-blue: #00f; }' +
+      '.some-class { color: tomato }' +
+      ':root { --color-green: #0f0; }',
+    ':root {' +
+      ' --color-red: #f00;' +
+      ' --color-green: #0f0; ' +
+      '}' +
+      ':root.blue { --color-blue: #00f; }' +
+      '.some-class { color: tomato }'
+  )
+})
+
+test('merges all :roots into one except :roots under at-rules', () => {
+  run(
+    ':root { --color-red: #f00; }' +
+      '@media screen { :root { --color-blue: #00f; } }' +
+      '.some-class { color: tomato }' +
+      ':root { --color-green: #0f0; }',
+    ':root {' +
+      ' --color-red: #f00;' +
+      ' --color-green: #0f0; ' +
+      '}' +
+      '@media screen { :root { --color-blue: #00f; } }' +
+      '.some-class { color: tomato }'
+  )
+})
diff --git a/web/test/vars-cleaner.test.ts b/web/test/vars-cleaner.test.ts
new file mode 100644
index 00000000..f4fe5a32
--- /dev/null
+++ b/web/test/vars-cleaner.test.ts
@@ -0,0 +1,62 @@
+import { equal } from 'node:assert'
+import { beforeEach, test } from 'node:test'
+import postcss from 'postcss'
+
+import {
+  getVarsCleanerError,
+  resetCleanerGlobals,
+  varsCleaner
+} from '../postcss/vars-cleaner.js'
+
+function run(input: string, output: string): string | undefined {
+  let result = postcss([varsCleaner]).process(input, { from: undefined })
+  equal(result.css, output)
+
+  return getVarsCleanerError()
+}
+
+beforeEach(() => {
+  resetCleanerGlobals()
+})
+
+test('clean unused palette colors', () => {
+  let error = run(
+    ':root {' +
+      '--red-100: #f00;' +
+      '--green-200: #0f0;' +
+      '--blue-300: #00f;' +
+      '}' +
+      '.selector {' +
+      ' color: var(--red-100)' +
+      '}',
+    ':root {' +
+      '--red-100:#f00;' +
+      '}' +
+      '.selector {' +
+      ' color: var(--red-100)' +
+      '}'
+  )
+
+  equal(error, undefined)
+})
+
+test('return error if unused css variables found', () => {
+  let error = run(
+    ':root {' +
+      '--used-variable: #00f;' +
+      '--unused-variable: #00f;' +
+      '}' +
+      '.selector {' +
+      ' color: var(--used-variable)' +
+      '}',
+    ':root {' +
+      '--used-variable:#00f;' +
+      '--unused-variable:#00f;' +
+      '}' +
+      '.selector {' +
+      ' color: var(--used-variable)' +
+      '}'
+  )
+
+  equal(error, 'Unused CSS variables: --unused-variable')
+})