I believe it would be a great addition if users were allowed to add CSS overrides for specific sites.
Basically, just a text box provide by selecting an option under Advanced Controls where the user could type or paste in some CSS that is applied to a specific site.
I think many users would find this feature very powerful (maybe hide it behind a flag so the power users who want it can use it, but that it doesn’t alter the experience for general users). Though it’s not like an extension, many user requests seen on here could be added by the user to have a similar functionality to what they have requested from the Brave team until extension support is added.
As an example, I myself have requested a feature to block Facebook from using user-select:none; on their mobile page. With this feature, it would be easy for me to fix.
I personally believe that this would give Brave for Android an edge over many other browsers with a powerful feature like this. Many power users could use this to enhance their Internet browsing experience drastically on sites where they have made poor choices with their CSS, and possibly allow a power user to even fix a broken site.