Dark Theme for the (rest of the) Web IDE
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In https://gitlab.com/gitlab-org/gitlab/issues/195701 we'll be supporting the `Dark` syntax highlighting preference for the code editing area in the Web IDE. We should expand the scope of the `Dark` preference to the rest of the Web IDE (sidebars, commit workflow, buttons, file tree, etc...).
**PREVIEW (Does not represent final):**

### Proposal
Version 1 of the designs is available here https://www.figma.com/file/1dJYV424qiSDVdmS5BKsNz/Dark-Web-IDE?node-id=93%3A218
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