Warn message should be more legible on light theme

[srirambv]

Description

Missed as part of #21053 verification, checksum invalid message is barely visible on light theme

Steps to Reproduce

1. Add an invalid checksum address in Send field
2. Error message is not visible at all when browser is set to light theme
3. Learn more should also have better top padding with the message

Actual result:

Light	Dark

Expected result:

More legible text on any theme

Reproduces how often:

Easy

Brave version (brave://version info)

Brave	1.43.89 Chromium: 105.0.5195.102 (Official Build) (64-bit)
Revision	4c16f5ffcc2da70ee2600d5db77bed423ac03a5a-refs/branch-heads/5195_55@{#4}
OS	Windows 11 Version 21H2 (Build 22000.918)

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 nightly channel? Yes

Other Additional Information:

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

Miscellaneous Information:

cc: @Douglashdaniel @supermassive @jamesmudgett

[srirambv]

Verification passed on

Brave	1.45.104 Chromium: 106.0.5249.126 (Official Build) beta (64-bit)
Revision	fb2e06d2dee20fe0c804528502b5ad72c613abd9-refs/branch-heads/5249_118@{#3}
OS	Linux

• Verified steps from issue description and brave/brave-core#15133
• Verified warning message has better legibility in light theme

Light	Dark

---

Verification passed on

Brave	1.45.104 Chromium: 106.0.5249.126 (Official Build) beta (64-bit)
Revision	fb2e06d2dee20fe0c804528502b5ad72c613abd9-refs/branch-heads/5249_118@{#3}
OS	Windows 11 Version 21H2 (Build 22000.978)

• Verified steps from issue description and brave/brave-core#15133
• Verified warning message has better legibility in light theme

Light	Dark

---

Verification passed on

Brave	1.45.104 Chromium: 106.0.5249.126 (Official Build) beta (arm64)
Revision	fb2e06d2dee20fe0c804528502b5ad72c613abd9-refs/branch-heads/5249_118@{#3}
OS	macOS Version 12.4 (Build 21F79)

• Verified steps from issue description and brave/brave-core#15133
• Verified warning message has better legibility in light theme

Light	Dark