The theme builder is found under your application Project in Go. Under the tab Themes.
You can either adjust a predefined theme (Generic/Material). Or select custom theme to upload your own theme.
Use the DevExtreme themebuilder to build a custom theme. https://devexpress.github.io/ThemeBuilder/
If you use a predefined theme you can adjust these settings.
Compact - Makes the theme more compact to fit more content in same resolution.
Accent Color - Sets accent color for the theme
Text color - Sets the standard text color
Background color - Sets the standard background colour.
Border color - Sets the standard border color
Border radius - Adjusts the boarder radius.
Logo file - Sets a logo file for your login screen.
Resource files - Upload resources files for your theme. I.e images.
Manifest json - Upload your Manifest json
Custom CSS - Add any custom CSS for your predefined theme or custom theme.
Save - Save and apply theme for all applications connected to your project.