Brave Welcome and Rewards pages - and others - have incorrect styling

[stephendonner]

Description

Brave Welcome and Rewards pages - and others - have incorrect border and button styling/colors/margins
Credit to @LaurenWags for the find(s); this is a regression from 1.26.19 and probably related to the Chromium bump to .57

Steps to Reproduce

1. clean profile
2. launch Brave
3. look at the brave://welcome, brave://rewards and other pages for rendering/CSS issues

Actual result:

There's an extra white border, fonts look weird, and margin spacing, etc. is off.

brave://welcome	brave://rewards

Expected result:

brave://welcome	brave://rewards

Reproduces how often:

100%

Brave version (brave://version info)

Brave	1.26.21 Chromium: 91.0.4472.57 (Official Build) nightly (x86_64)
Revision	e3443317fa07f1e9997e4a9c738eddfefc3c0292-refs/branch-heads/4472_54@{#6}
OS	macOS Version 11.3.1 (Build 20E241)

Version/Channel Information:

• Can you reproduce this issue with the current release? no
• Can you reproduce this issue with the beta channel? no
• Can you reproduce this issue with the nightly channel? yes

/cc @mkarolin @rebron @brave/legacy_qa

[stephendonner]

Also the interstitial Brave IPFS page (test with new profile on ipfs://brantly.eth)

broken	expected

[stephendonner]

@zenparsing can/should this be slotted for 1.26.x? Thanks!

[stephendonner]

Verified FIXED using the inline testplan on build

Brave	1.26.30 Chromium: 91.0.4472.57 (Official Build) nightly (x86_64)
Revision	e3443317fa07f1e9997e4a9c738eddfefc3c0292-refs/branch-heads/4472_54@{#6}
OS	macOS Version 11.3.1 (Build 20E241)

Steps:

1. new profile, launch Brave
2. check brave://welcome and brave://rewards layouts

Confirmed no visible CSS regressions (no white border, fonts are normal again, etc.)

brave://welcome	brave://rewards

#15954 is being taken care of separately for the IPFS interstitial page 👍