Use opaque color for tab background in dark mode tab strip

[sangwoo108]

When the color has opacity, the hover glow effect's color will be multiplied, which ends up with too strong effect. In order to avoid that use opaque color by blending tab's background and tab strip background color.

Resolves brave/brave-browser#29436

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• 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 lint, 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/Custom-Headers
  • 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:

[stephendonner]

Verification PASSED using

Brave	1.52.25 Chromium: 112.0.5615.49 (Official Build) nightly (x86_64)
Revision	bd2a7bcb881c11e8cfe3078709382934e3916914-refs/branch-heads/5615@{#936}
OS	macOS Version 11.7.5 (Build 20G1225)

First, reproduced the heavy hover effect using 1.51.79:

Brave	1.51.79 Chromium: 112.0.5615.49 (Official Build) beta (x86_64)
Revision	bd2a7bcb881c11e8cfe3078709382934e3916914-refs/branch-heads/5615@{#936}
OS	macOS Version 11.7.5 (Build 20G1225)

Steps:

0. set macOS to Dark Mode
1. installed 1.151.79
2. launched Brave
3. loaded a couple pages
4. context-clicked on the tab's titlebar
5. chose Use vertical tabs
6. noted default (unselected) state and on hover states for tabs

unselected, default state	on hover (heavier)

Confirmed FIXED using 1.52.25:

Steps:

1. installed 1.152.25
2. launched Brave
3. loaded a couple pages
4. context-clicked on the tab's titlebar
5. chose Use vertical tabs
6. noted default (unselected) state and on hover states for tabs

Confirmed a lighter hover-state effect, matching that in brave/brave-browser#29436 (comment) 👍

unselected, default state	on hover (lighter)