[Nala]: Fix Omnibox highlights and vertical tab separator with custom dark theme

[fallaciousreasoning]

Resolves brave/brave-browser#40971
Resolves brave/brave-browser#41055

Submitter Checklist:

• I confirm that no security/privacy review is needed and no other type of reviews are needed, or that I have requested them
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally:
  • npm run test -- brave_browser_tests, npm run test -- brave_unit_tests wiki
  • npm run presubmit wiki, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

1. In the light theme
2. Disable brave://flags/#brave-web-view-rounded-corners
3. Install https://chromewebstore.google.com/detail/just-black/aghfnjkcakhmadgdomlmlhhaocbkloab
4. Type in the omnibox
5. Selected result should not be white ish
6. Hovering over results should not be white ish
7. Separator above Ask Leo should not be white ish
8. View a dark page - there should be a separator between vertical tabs and the page - it should not be white

[fallaciousreasoning]

We now take the Chromium colors which is much simpler - we still need this for Private/Tor

[fallaciousreasoning]

Its not safe to use Nala theme variables without checking for a custom theme, unfortunately 😢

[MadhaviSeelam]

Verification PASSED using

Brave | 1.72.22 Chromium: 129.0.6668.59 (Official Build) nightly (64-bit)
-- | --
Revision | a2054d5f1e83d781a52e1baee7a2f3307250c1d6
OS | Windows 11 Version 23H2 (Build 22631.4169)

Reproduced the issue in 1.72.3

example	example

Light theme

1. Installed 1.72.22
2. launched Brave
3. kept Windows in light theme mode
4. verified the flag brave://flags/#brave-web-view-rounded-corners is disabled
5. installed https://chromewebstore.google.com/detail/just-black/aghfnjkcakhmadgdomlmlhhaocbkloab
6. typed London in the omnibox

• Confirmed Selected result is not white ish
• Hovering over results should not be white ish
• Separator above Ask Leo should not be white ish

example	example	example	example	example

Dark theme

1. Installed 1.72.22
2. launched Brave
3. switched Windows theme to dark mode
4. verified the flag brave://flags/#brave-web-view-rounded-corners is disabled
5. typed London in the omnibox

Confirmed Selected result is not white ish

Hovering over results should not be white ish

Separator above Ask Leo should not be white ish

Confirmed a separator between vertical tabs and the page is shown and it is not white

example	example	example