Dark Theme for the (rest of the) Web IDE

In #195701 (closed) 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