Custom theme support gives you almost unlimited control over the look-and-feel of any presentation.
This feature is powered by Cascading Style Sheets aka. CSS. As a presentation author you can define and activate new CSS style rules that override the default style rules inherited from the fixed theme chosen for your presentation.
To apply your own custom CSS styles to your slideshow presentation, see the custom theme setting docs.
To learn what styles are defined your chosen theme and by extension what styles you can override for you presentation, jump straight to the CSS files on GitHub. You can find those files here.
While studying the CSS files that define the rules for your chosen theme is a great way to familiarize yourself with the styles available, using a Web Inspector to directly inspect the content on your slides is highly recommended.
A Web Inspector - typically a Developer Tools feature in your browser - provides a highly interactive and very effective way to discover active style rules on any element on your slide. Plus it also gives you an opportunity to experiment with tweaks and overrides and see the results instantly updated in your slideshow within the browser window.
Once you have achieved the effects you are after using the Web Inspector you can then capture the new rules in your own custom CSS file. To then activate your custom CSS, see the custom theme setting docs.