Customize slides (styles)

What I’m trying to do

Hello gang,

I’m trying to use the core Slides feature.
It works, but I can’t find a way to customize the rendering of the slides.

For instance, I’d like to have everything but headers align-left.

Is there a css file I can add or edit?
Is there a menu I’m missing?

The default style is quite… raw.

I’ve been considering using the community plugin “advanced slides” instead.
Maybe that’s the way to go?

Thanks for your help!