Allow theme dropdown in onboarding flow

[srirambv]

Description

As @rossmoody mentioned in #4992 (comment) there should be a dropdown option

Steps to Reproduce

Actual result:

Expected result:

Stole @rossmoody image from #4992 (comment)

In the case of themes the user can see their theme change in real time so that’s the confirmation.
Onboarding flow should also has a dark mode( #3911) which can be a good indication of how it looks when dark mode is enabled.

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	0.68.113 Chromium: 76.0.3809.62 (Official Build) beta (64-bit)
Revision	7b77856b3aa34d72f246d12340fc1ded8b2c0e83-refs/branch-heads/3809@{#798}
OS	Linux

Version/Channel Information:

• Can you reproduce this issue with the current release?
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the dev channel?
• Can you reproduce this issue with the nightly channel?

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields?
• Does the issue resolve itself when disabling Brave Rewards?
• Is the issue reproducible on the latest version of Chrome?

Miscellaneous Information:

cc: @rossmoody @rebron

[kjozwiak]

Isn't this already implemented in nightly via brave/brave-core#2909? Checked the the latest nightly and see the following using 0.70.18 Chromium: 76.0.3809.72:

@srirambv am I missing something?

[srirambv]

#4992 didn't have the actual implementation which is why i logged this issue. I had not checked the PR which had the new implementation.