This is a feature available to Chalk Gold subscribers and teachers whose schools have a paid subscribed to Chalk. You will not have this option if you are a free user.
Subscribing to Chalk Gold, Teams, or a Chalk solution for schools will give you access to the "Themes" feature, which allows you to customize your Chalk apps with optional provided colour palettes.
Click the profile icon in the top-right corner of the page, then click Theme.
Click on one of the options from the Theme page.
The header and sub-header colours in the top navigation menu will be changed to the option you selected.