You can upload your own custom CSS to affect how your Teamwork Projects site looks.

Note - this feature is only available on the Professional and Business plans.

To enable the feature, first just choose a custom colour theme, then go to the 'Integrations' area of the 'Settings' menu in the top right of your screen. In this section, you'll see the option for 'Custom Styles':

If you click the 'Manage CSS' button you'll see a screen where you can select 'Enable custom CSS for my account' and then you can paste your code into the text area: