Adding theme support to organization navbar items background color

[HiDeoo]

Following the recent layout UI updates, it would be nice if the background color of the navbar items was customizable through themes.

At the moment, the background color is hardcoded to linear-gradient(137.34deg, #4000BF 4.44%, #154B62 95.33%) in

insomnia/packages/insomnia/src/ui/components/organizations-navbar.tsx

Line 48 in 45704e4

background: 'linear-gradient(137.34deg, #4000BF 4.44%, #154B62 95.33%)',