[Desktop] GitHub favicon does not display on panel/rewards page when using dark theme

[LaurenWags]

Description

If your install is using the "dark" theme, the standard GitHub favicon will not display on the panel or in brave://rewards.

This is similar to #9030 which noted this for the NTP top site tiles. On that issue, there's a comment noting that GH favicons are theme-aware, #9030 (comment). Logging this issue for Rewards as I didn't want to assume any work done with that issue would carry over to rewards pages.

Steps to Reproduce

1. Use dark theme and have rewards enabled
2. Navigate to a GH publisher (non-verified is best so you get the default favicon)
3. Open panel

Actual result:

Looks like no favicon

Expected result:

GH favicon to be displayed

Reproduces how often:

easily

Brave version (brave://version info)

1.17.x

Version/Channel Information:

• Can you reproduce this issue with the current release? yes, 1.16.x
• Can you reproduce this issue with the beta channel? yes, 1.17.x
• Can you reproduce this issue with the nightly channel? yes, 1.18.x

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:

[Miyayes]

Brave	1.51.118 Chromium: 113.0.5672.126 (Official Build) (x86_64)
Revision	c541687b21a73452ab403e2dced7033ddc97ee9d-refs/branch-heads/5672@{#1202}