.page_main {
    background-color: rgba(var(--theme-page-background-color--rgb), 0.8);
}

Did not work :(

My bad. Looks like there are two underscores in the class for some odd reason. Try adding another underscore so it looks like this.

.page__main {
    background-color: rgba(var(--theme-page-background-color--rgb), 0.8);
}

"Warning: Expected (<color> | inherit) but found 'rgba(var(--theme-page-background-color--rgb) , 0.8)'." this is dumb

Just ignore it.

Those kind of warnings are a long lasting bug of MediaWiki's CSS editor due to the editor's syntax check being outdated and not recognizing css variables (and a few other newer css rules and notation forms).

See also "Common issues" here: Help: Advanced CSS and JS#Common issues