[Desktop] Web Extension Inconsistency. Simple / Mystery

[ImprovedTube]

Description

A simple Extension feature wont work consistently for no apparent reason, as it works in Google Chrome in every OS
Our themes working in Brave are same to those that dont work. code-charity/youtube#521

Steps to Reproduce

Enable Improvedtube Youtube Themes:
https://chrome.google.com/webstore/detail/improve-youtube-open-sour/bnomihfieiccainjcjblhegjgglakjdd
Edit: Only Desert & Plain don't work. Thats the Mystery

Actual result: -

Expected result:

Themes all working in exactly the same way.
(Night, Dawn, Sunset, Desert, Plain, Black.
(Except 'Dark', which refers to Youtube's night mode )

Reproduces how often:

always

Brave version (brave://version info)

84.0.4147.89 - 84.0.4147.135

Version/Channel Information:

• Can you reproduce this issue with the current release? YES instantly reproduced. Takes you few clicks
• Can you reproduce this issue with the beta channel?
• Can you reproduce this issue with the nightly channel?

Other Additional Information:

• Does the issue resolve itself when disabling Brave Shields? NO
• Does the issue resolve itself when disabling Brave Rewards? NO
• Is the issue reproducible on the latest version of Chrome? Chromium? (Google Chrome: NO)

Miscellaneous Information:

[rebron]

@ImprovedTube Are you still seeing this issue with 1.15.75 Chromium 86?

[rebron]

@ImprovedTube Looks like this is working except for maybe two of the styles? Please add some information on expected outcome, if you'd like us to take a look.

[ImprovedTube]

thank you @rebron !
did you click all themes, desert, plain?
happens in Opera too according to our user. "it doesn't seem to be fixed on my end" https://i.vgy.me/Iw9Jms.webm
"Tested on the latest versions of Opera GX and Brave, on Windows 10 Home. Also tested on Ubuntu 20.04 with Brave."
( code-charity/youtube#521 (comment) )