Rework themes for parity with the old nav
### Problem to solve
We've received numerous requests about missing the theme options available in the old navigation. These themes served as a simple way to distinguish GitLab from other webpages, and without them it can be hard to swap between different instances.
### Proposal
Each available theme should have a distinct visual representation in the new navigation.

 [**View Figma →**](https://www.figma.com/file/jp4vmGJIlhSkvsy4JisduQ/%23378549-Rework-themes-for-parity-with-the-old-nav?type=design&node-id=122%3A86699&mode=design&t=7FTmyYyKV3Ywrohu-1)
### Implementation specs
Each of the colored theme will use a similar scale of color variables.
| Theme | Scale | Sidebar border color |
|-------|-------|----------------------|
| Gray | `$gray-900/100/50` | `$gray-950 8%` |
| Indigo | `$t-indigo-900/100/50` | `$gray-950 8%` |
| Light Indigo | `$t-indigo-700/100/50` | `$gray-950 8%` |
| Blue | `$t-blue-900/100/50` | `$gray-950 8%` |
| Light Blue | `$t-light-blue-700/100/50` | `$gray-950 8%` |
| Green | `$t-green-900/100/50` | `$gray-950 8%` |
| Light green | `$t-green-700/100/50` | `$gray-950 8%` |
| Red | `$t-red-900/100/50` | `$gray-950 8%` |
| Light Red | `$t-light-red-700/100/50` | `$gray-950 8%` |
### Data
[Current theme usage](https://app.periscopedata.com/app/gitlab/695468/User-preferences-(90-day-active-users)?widget=10039861&udv=0)
<details>
<summary>Snapshot</summary>

</details>
### What does success look like, and how can we measure that?
issue