Oh I didn’t realize Advanced Slides doesn’t use vault CSS snippets. Took a quick look at the plugin and presume you already know about
larger
<!-- element style="font-size:1.5em" -->
but don’t like it because it renders in only Advanced Slides, not in Slides or the note.
If it’s that you want it to work in all three and with markdown, then you might have to use both snippets and Advanced Slides’ Element Annotations on each styled element in your slide.
Or see if the successor plugin Slides Extended handles it better, since Advanced Slides was discontinued a while ago.
Or consider doing all HTML without trying to mix with markdown. Wouldn’t be my first choice, but does render in all three.