Fix BraveThemeServiceTest failure

[simonhong]

Recently, frame colors were changed.
Failure reason is this test uses hard-coded color value.
As a todo list, we should share these color constants instead of copying.

Issue brave/brave-browser#961

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Needed or QA/No-QA-Needed) to include the closed issue in milestone

Test Plan:

yarn test brave_browser_tests --filter=BraveThemeServiceTest.BraveThemeChangetest

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[simonhong]

Closed by #482