Navigation - Add option to change navigation color palette
Previous versions of GitLab allowed users to customize the navigation sidebar with a color theme. This was an appreciated feature, and users could also use color to differentiate GitLab instances.
With the change to our new navigation, we have a chance to bring back this feature. Our default palette is an indigo one, based on the GitLab identity, but we should allow users to choose from a variety of themes.
These themes should be mindful of our current use of color as to not diminish its meaning and intention (e.g. red is used for destructive actions.)
Revert !9199 (merged) and apply the following changes and colors:
For the Preferences screen, show the theme options in the same style as the syntax highlighting themes, with the following description:
Customize the appearance of the application header and navigation sidebar.
For the squares, use the values specified in the following tables: