In the admin it is possible to create custom themes for templates.
For the styles to have effect you will need to use the #SLIDE_ID to all styling.
#SLIDE_ID will be replaced with an actual id in the Client to isolate the styling on the current slide.
An example file can be found in this directory: example.css.
Below is an example of variables on the slide container.
#SLIDE_ID {
/* Default variables */
--bg-light: #f6f6f6;
--text-dark: #333333;
--color-primary: #3761d9;
--color-secondary: #ef0043;
--font-family-base: Arial, "sans-serif";
/* Darkmode overrides */
--bg-dark: #333333;
--text-light: #ffffff;
}Below is a example of simple background color on the whole slide using the template Text and Image.
#SLIDE_ID .template-text-image {
background-color: hsl(240, 100, 99);
}Below is a example of setting a color on H1 elements.
#SLIDE_ID h1 {
color: hsl(240, 3, 7);
}