If you have some knowledge about CSS, you can download the theme's .zip package from the Themes and Brands section inside the System Administrator tab. The theme is split into several .css files. Depending on the theme's version, the files with most of the theme's code are app_nav.css, theme_specific.css and sp8.css.
If you don't wanna fiddle with the original code, you can create a new custom.css file and add an import inside theme.css.
From there, you can use the standard web inspector from a web browser to get the right css rule you wanna customize and simply save and load this new .zip into a new theme in the Themes and Brands section of your Learn's Administrator panel.
Or at least that's the way I do it, don't know if there's any official documentation.