brave://welcome Next/Done button doesn't have Brave style - follow up to 6398

[LaurenWags]

Description

Found while testing #6398

The Next button on the brave://welcome page does not use the Brave style from above issue.

Steps to Reproduce

1. Launch brave://welcome page. Click on Let's go.
2. View Next button
3. Hover over Next button
4. Click on Next button
5. When you get to the end of the flow, note the same happens for Done button.

Actual result:

button doesn't have Brave styling

Dark/Light display of button (no interaction):

Dark/Light hover over button:

Dark/Light click on button:

Expected result:

Brave styling as in #6398

Reproduces how often:

easily

Brave version (brave://version info)

Brave	0.71.112 Chromium: 78.0.3904.87 (Official Build) beta (64-bit)
Revision	20c21f4010010f32462ea8e1d6af30cef66d48c8-refs/branch-heads/3904@{#840}
OS	macOS Version 10.13.6 (Build 17G5019)

Version/Channel Information:

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

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:

[LaurenWags]

Duplicate of sub item in #6707