Clean up `<code>` styles

Created by: sqs

Add a CSS var --code-bg that is used in CodeSnippet and Markdown code blocks. (Because it's used in 2 places, a CSS var is used instead of just --color-bg-2.)

Also fix a bug where the CodeExcerpt style was not actually applied in the storybook because only the global stylesheet was being loaded.

Created by: sourcegraph-bot

Notifying subscribers in CODENOTIFY files for diff d85e8e1ddb72d334e9586fdd3e0aa3785b173a75...b126254f05704fc81b689cd934c28f025da105de.

Notify	File(s)
@LawnGnome	client/web/src/enterprise/campaigns/detail/snapshots/CampaignSpecTab.test.tsx.snap
@eseliger	client/web/src/enterprise/campaigns/detail/snapshots/CampaignSpecTab.test.tsx.snap

Created by: codecov[bot]

Codecov Report

Merging #14817 into main will decrease coverage by 0.00%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main   #14817      +/-   ##
==========================================
- Coverage   52.20%   52.19%   -0.01%     
==========================================
  Files        1556     1556              
  Lines       79513    79513              
  Branches     7145     7076      -69     
==========================================
- Hits        41508    41504       -4     
- Misses      34256    34260       +4     
  Partials     3749     3749              

Flag	Coverage Δ
#go	52.39% <ø> (-0.01%)
#integration	30.34% <ø> (-0.02%)
#storybook	22.19% <ø> (ø)
#typescript	51.71% <ø> (-0.02%)
#unit	33.92% <ø> (ø)

Impacted Files	Coverage Δ
client/branded/src/components/CodeSnippet.tsx	100.00% <ø> (ø)
client/web/src/settings/SettingsPage.tsx	64.70% <0.00%> (-11.77%)
client/web/src/tree/TreeRoot.tsx	79.31% <0.00%> (-1.73%)
cmd/repo-updater/repos/types.go	74.20% <0.00%> (-0.25%)