You can use custom CSS stylesheets to change a look of workspace indicator widget.
As a base, use stylesheet.css, as it has all classes needed to create a custom stylesheet for Improved Workspace Indicator.
Note Creating custom CSS stylesheets might require from you some actual knowledge of CSS. You can check this tutorial on MDN to learn basics of CSS.
Which parts of a stylesheet you would want to modify:
.panel-workspace-indicator,
.panel-workspace-indicator-box .workspace {
border: 1px solid #cccccc;
background-color: rgba(200, 200, 200, 0.5);
}
Located in lines 13-17
In this part you can customize a look of a single workspace indicator, as well as colors for an inactive workspace.
.workspace-indicator-active {
background-color: rgba(20, 200, 200, 0.5);
}
Located in lines 28-30
Here you can change properties of an active workspace indicator.
Open extension's preferences and locate Custom CSS stylesheet
row. Now either click on folder button which will open a file chooser dialog, or paste a path of your stylesheet file into textbox.
Close preferences window and re-enable extension by toggling an switch in extensions manager.
Note You can disable custom CSS by removing an path from textbox in preferences window.