There’s the option to use the API to customize the CSS style, see Settings and configuration - DHIS2 Documentation; however, it seems that currently the CSS is loaded only for the login page and then it’s not taking effect (UI Customization (style.css) not taking effect?.
I think it is possible to make changes using a custom app but I have not tried it. The reason I think it would work is because the Training App app seems to work at a global level.