Being able to change the user interface theme is one of the major features of Saltcorn.
The Saltcorn user interface is based on Boostrap 4. This is a system which allows you to edit the CSS (Cascading Style Sheets) which gives you application a unique look and feel.
In Saltcorn go to Settings / Plugin and pack store
In the Plugin page > Themes
I would recommend starting with any-bootstrap-theme.
This will give you a lot of options for different bootstrap-based themes.
Click Install, and select a theme from a drop-down and change some parameters.
Return to this page by clicking the settings cog icon in the any-bootstrap-theme card in the plug-in store (when you have it installed it is available under the Installed tab at the top next to themes.)
You can also try some of the other themes instead, adminlte is not really maintained but the other ones should work.
To set a different theme for every role, go to Settings > Users and security > Roles, and choose the theme you want for each role.
The any-bootstrap-theme plug-in has an option to use and uploaded file as the stylesheet. You can use this to customise every aspect of the look and feel of your application. Steps: