Added system default theme mode on linux

[simonhong]

Resolves brave/brave-browser#14685

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 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:

1. Launch brave and set Same as Linux theme settings option.
2. Change to Dark or Light and check browser theme is changed properly
3. Change to Same as Linux
4. Change linux system theme. Ex, gnome tweak application can change system theme on Ubuntu.
5. Check browser theme is changed for system theme change
6. Change to Dark or Light
7. Change linux system theme and check browser theme is not changed
8. Change to Same as Linux
9. Re-launch browser and check browser theme has same theme with system theme

[fallaciousreasoning]

LGTM

[goodov]

chromium_src lgtm

[kjozwiak]

Verification PASSED on PopOS 22.04 x64 using the following build(s):

Brave | 1.62.89 Chromium: 120.0.6099.35 (Official Build) nightly (64-bit)
--- | ---
Revision | f68a18538c0af7dba0f91c6176be4c1eee17101c
OS | Linux

Using the STR/Cases outlined via brave/brave-browser#14685 (comment), reproduced the original issue using 1.60.118 Chromium 119.0.6045.163 as per the following:

Example	Example

Using the STR/Cases outlined via #21065 (comment), ensured that the fix/changes were working as per the following:

Same as Linux (Light Theme)

Example	Example

Same as Linux (Dark Theme)

Example	Example

Dark Theme (Linux theme set as Light)

Example	Example

Light Theme (Linux theme set as Dark)

Example	Example