It is possible to customize themes, or even to create your own from scratch, with Mautic. To do this go to the Theme Manager, open the drop down menu next to the pre-installed theme you want to modify and download it.
To customize the downloaded theme, unzip the files to an empty folder. Name the folder as the new name of your theme. Then edit the following files to amend the theme paths and name:
Mautic themes are written in HTML and TWIG, to make amendments to the structure or layout simply edit the files in the new theme and upload them to your instance. Ensure that your hosting provider does not have caching enabled, as this can sometimes prevent changes to CSS files being replicated instantly.