Identity Portal doesn't support branding for apps, you can still add custom rules in the branding from Admin UI to style the elements like desired.
1. Find the selector by inspecting the element using developer tools ( .links-container .link-category .category-name is the selector for the header in apps screen)
2. Go to admin UI, Branding Tab.
3. Create a custom CSS selector by clicking on the '+' icon.
4. Give the name as the selector we saved in the earlier step. (step1)
5. Click add an attribute and select color from the list.
6. add the value as desired.
7. Save it.
8. Refresh Apps screen in Sigma UI.