You can customize how your filesystem looks. The colours chosen here apply to the directory you're currently in and all files and directories in this directory. Colours which you do not want to modify can be left empty. Then the default theme colours will be used.
The highlight colour is used for highlighting selected buttons and other elements. It's also used as the page's theme colour, this affects things like the embed colour in Discord and the colour of the address bar in some web browsers.
You can choose an image to show above or behind the files in this directory. The image will be picked from your filesystem. The image will get a shared file link. You can move and rename the file like normal, but if you remove the shared file property your branding will stop working. Recommended dimensions for the header image are 1000x90 px.
Below is an example of what the site looks like with these colours: