The infobars have a light/dark colour scheme, but this follows the OS-level preference, not the theme's. UX would prefer that we match the theme's preference (this is brighttext in the code) rather than the OS.

STR

1. Set your theme to light or dark, opposite your OS preference
2. Trigger an infobar

• Go to https://permission.site/
• Click "Protocol Handler"

Expected results: The infobar colours match your theme's dark/light
Actual results: The infobar is light on dark or dark on light, matching the OS

I separated out the prefers-color-scheme: dark in common.inc.css so that we can a version with that and without that and use the version without that for infobars. Doing this eliminates the OS theme changes affect on the infobar. Unfortunately, it's still not perfect. The button styling seems to be off.

This also affects proton modals and infobars and whatever else
includes common.css in its shadow DOM in browser chrome.

Promoted to P1 after discussions in Slack surrounding MR1 triage.

Depends on D111486

Depends on D111970

Depends on D111971

Depends on D111972

Depends on D111973

Depends on D111974

Depends on D111975

Depends on D111976

Depends on D111977

Depends on D111486

Backed out for failures on browser_toolbar_prefers_color_scheme.js

backout: https://hg.mozilla.org/integration/autoland/rev/48a99646f183abc34e8eeb695fcede5bcc9463d1

push: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=Chl6doGmSeCF0Amm7_iYUQ.0&revision=79c1392f7c4303028fce92d68d399af21bd75a27&group_state=expanded

tier 1 failure seen on the next push: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=EqOo_4CkQduiGAkQZyvnsg.0&revision=f27f2a126a3a95f160ded1dc2c5802e44e964f52&searchStr=windows%2C10%2Cx64%2Copt%2Cmochitests%2Ctest-windows10-64%2Fopt-mochitest-browser-chrome-e10s%2Cbc7

failure log: https://treeherder.mozilla.org/logviewer?job_id=336694077&repo=autoland&lineNumber=3100

[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - TEST-PASS | browser/components/extensions/test/browser/browser_toolbar_prefers_color_scheme.js | OS dark mode unchanged -
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - Buffered messages finished
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - TEST-UNEXPECTED-FAIL | browser/components/extensions/test/browser/browser_toolbar_prefers_color_scheme.js | toolbar query is light mode -
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - Stack trace:
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - chrome://mochikit/content/browser-test.js:test_ok:1334
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - chrome://mochitests/content/browser/browser/components/extensions/test/browser/browser_toolbar_prefers_color_scheme.js:testThemeDeterminesToolbarQuery:47
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - Leaving test bound testThemeDeterminesToolbarQuery
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - GECKO(3437) | MEMORY STAT | vsize 3114MB | residentFast 444MB | heapAllocated 200MB
[task 2021-04-15T19:01:38.396Z] 19:01:38 INFO - TEST-OK | browser/components/extensions/test/browser/browser_toolbar_prefers_color_scheme.js | took 418ms

Backed out changeset 1c9943dacbdb (Bug 1701691) for causing failures in browser_toolbar_prefers_color_scheme.js CLOSED TREE

Range: https://treeherder.mozilla.org/jobs?repo=autoland&group_state=expanded&collapsedPushes=842382&resultStatus=success%2Ctestfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&fromchange=d26e6241a27381f7d94a055d1dc8297a9d7a7fc2&searchStr=os%2Cx%2C10.14%2Cwebrender%2Cdebug%2Cmochitests%2Ctest-macosx1014-64-qr%2Fdebug-mochitest-browser-chrome-e10s%2Cbc7&tochange=35d7939ec0892e4d041c82606bcff69a0d5f0f15

The job didn't run on the original push because of some build bustages from another bug.

Log: https://treeherder.mozilla.org/logviewer?job_id=336747500&repo=autoland&lineNumber=30454

Backout: https://hg.mozilla.org/integration/autoland/rev/163d8a588c39ed388181a2b202c5235f95167adf

https://hg.mozilla.org/mozilla-central/rev/7e864fb91533
https://hg.mozilla.org/mozilla-central/rev/86d38280fedc
https://hg.mozilla.org/mozilla-central/rev/e05824608cb9
https://hg.mozilla.org/mozilla-central/rev/4c299a07326b
https://hg.mozilla.org/mozilla-central/rev/07d036bff2a2
https://hg.mozilla.org/mozilla-central/rev/fa6845ddd370
https://hg.mozilla.org/mozilla-central/rev/69ffe4edadc1
https://hg.mozilla.org/mozilla-central/rev/054f9a3d20b1
https://hg.mozilla.org/mozilla-central/rev/5a498c95f727

Verified the fix on MacOS 11, Windows 10 and Ubuntu 20.04 using Firefox Nightly 90.0a1 (20210419221626)

Updating the description, since this updated our common styling. This updates the in-content pages (about:preferences, about:addons, etc that use our common styling), infobars, alerts, pdfjs, print modal, and the webrtc indicator (maybe not a definitive list). In general, if it's Firefox UI and the theme is "dark" then that Firefox UI should now be dark as well, rather than following the OS.