Update welcome experience layout

[rossmoody]

Need to open up the Welcome Experience layout and break it out of the modal cards. This will help with dark mode adoption, responsiveness, larger communication and gives us more room to breathe on smaller screens.

[srirambv]

Personally I feel the dark mode looks really harsh on screen.There are way too many dark elements and the orange button on dark background looks really harsh. Instead of black background we can probably go with softer gray colour which would still give the dark feel but not as bad as it looks in the screenshot.

[GeetaSarvadnya]

Verification passed on

Brave	0.69.128 Chromium: 77.0.3865.75 (Official Build) (64-bit)
Revision	201e747d032611c5f2785cae06e894cf85be7f8a-refs/branch-heads/3865@{#776}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified the test plan from Allow theme change during Welcome onboarding brave-core#2909
  

Verification passed on

Brave	0.69.128 Chromium: 77.0.3865.75 (Official Build) (64-bit)
Revision	201e747d032611c5f2785cae06e894cf85be7f8a-refs/branch-heads/3865@{#776}
OS	Ubuntu 18.04 LTS

• Verified the test plan from Allow theme change during Welcome onboarding brave-core#2909
• Selecting a theme from the onboarding flow doesn't open brave://setting page in a new tab
  

Verification PASSED on macOS 10.14.6 x64 using the following build:

Brave	0.69.129 Chromium: 77.0.3865.90 (Official Build) (64-bit)
Revision	58c425ba843df2918d9d4b409331972646c393dd-refs/branch-heads/3865@{#830}
OS	macOS Version 10.14.6 (Build 18G95)

• Verified using the test case outlined under Allow theme change during Welcome onboarding brave-core#2909