Dark mode online judge

[int-y1]

This is for the many users who requested dark mode. This issue is possibly a subset of #578.

A few general guidelines:

• I will try to keep this description up to date. For any bugs that I missed, feel free to post them in this issue.
• Feel free to make a PR for any of these bugs. A few tips to make review easier:
  • If you're working with SCSS, you'll find these files useful: /resources/vars-*.scss
  • You can put Part of #2035 in your PR description. Don't put Closes #2035/Fixes #2035.
  • Post a screenshot of before change + after change.
  • For small changes, average review time is ~2 days. For big changes, review will take longer.

Important styling bugs:

• https://dmoj.ca/problem/2dperm Blockquote sidebar is too dark. (Thanks, @Xyene)
• https://dmoj.ca/edit/profile/ The user script editor's header and sidebars are light.

Less important styling bugs:

• https://dmoj.ca/stats/language/?dark Pie chart's labels are black.
• When hovering over the X on alerts (e.g., https://dmoj.ca/problem/cco21p6/tickets/new), the X blends in to the background. It's also pretty light to begin with. (Thanks, @Xyene)
• Images need background-color: white; because the image may have transparency.
• https://dmoj.ca/edit/profile/ Set site theme to dark and disable experimental features. The editor uses dark mode. (Thanks, @Riolku)
• Soften whites and blacks (see @Xyene's comment)

When you're bored:

• Go to /edit/profile/, enable experimental features, and set site theme to dark. Visit any page, and try to find styling bugs.
• Find page-specific <style> with git grep -n "<style". At a minimum, migrate the color-specific properties into scss file(s).
• Import more ace.js themes from upstream (see @Xyene's comment).

[Xyene]

• When hovering over the X on alerts (e.g., https://dmoj.ca/problem/cco21p6/tickets/new), the X blends in to the background. It's also pretty light to begin with.
• We should soften whites and blacks, e.g.

instead of

Headers should be brighter than the content they are heading.