Allow theme change during Welcome onboarding

[bsclifton]

Fixes brave/brave-browser#4992
Closes brave/brave-browser#4657
Closes brave/brave-browser#3911

Brave-ui (Dependency): brave/brave-ui#487

Quick view:

• 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).
• Verified that these changes build without errors on
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• 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/Yes or QA/No) to include the closed issue in milestone
• Public documentation has been updated as necessary. 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-Protection-Mode
  • 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-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

• welcome page displays in dark mode/light mode/system depending on user settings
• the theme step of the welcome page has a confirm button that is disabled unless valid option is selected in the dropdown
• selecting a valid option in the dropdown will change browser theme accordingly
• selecting confirm will go navigate to the next step of on-boarding
• On operating systems that don't support dark mode, the option (System theme (default)) will not show up in the dropdown

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

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.

[bsclifton]

On any Web UI page, you can demo the get (was already present) and the set (which was added by my commit) by opening the console and trying the following:

chrome.braveTheme.getBraveThemeList(function (types) { console.log(types)})
> undefined
VM219:1 [{"name":"Same as macOS","value":0},{"name":"Dark","value":1},{"name":"Light","value":2}]

chrome.braveTheme.setBraveThemeType('Light')
> undefined // theme will change to light mode

chrome.braveTheme.setBraveThemeType('Dark')
> undefined // theme will change to dark mode

chrome.braveTheme.setBraveThemeType('LOLOL')
> undefined // theme will change to "default"

[cezaraugusto]

++

[cezaraugusto]

[imptrx]

Merging in as:

• CI passed all tests but was failing/unstable due to dependency audit issue unrelated to this PR from rebase on July 25
• CI is failing from rebase on July 26 due to a version mismatch between brave-browser and brave-core which is also unrelated to the changes here

[petemill]

Does this drop down still show System theme on OS which does not have system theme (Win10 before 2019, Win8/7, macOS before 2018, Linux)? The purpose of the API which retrieved the list of themes is to make the decision about which are supported on the current system. We should probably use that same code path. Plus, if we’re intending to change the language from Same as xyzOS to System Theme, then we should probably be consistent about that in both Welcome and Settings UI.

[imptrx]

@petemill This drop down won't show the Same as xyzOS/System theme if system doesn't support it - I extrapolated that detail from the array length from the getBraveThemeList API

cc: @rossmoody @rebron for thought on parity between the dropdowns for on-boarding and settings page