Allow configuring ColorMode and ColorScheme in gitlab.yml.erb

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What does this MR do?

Allows to set a ColorMode (dark-mode) and a ColorScheme (syntax highlighting theme) in /etc/gitlab/gitlab.rb for omnibus installations:

gitlab_rails['gitlab_default_color_mode'] = 2
gitlab_rails['gitlab_default_syntax_highlighting_theme'] = 3

The updated template for the final gitlab.yml didn't include those config values before, although the gitlab codebase is able to work with the, after merging gitlab!141459 (merged) and gitlab!161325 (merged).

It should be possible to set these settings for docker(compose) installations like so too:

version: '3'
services:
  gitlab:
    ...
    environment:
      GITLAB_OMNIBUS_CONFIG: |
        gitlab_rails['gitlab_default_color_mode'] = 2
        gitlab_rails['gitlab_default_syntax_highlighting_theme'] = 5
        ...

Related issues

• not an Issue but a discussion: https://forum.gitlab.com/t/dark-mode-for-non-logged-in-viewer/100953/5

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