Being able to change the user interface theme is one of the major features of Saltcorn.
In an open Saltcorn instance, go to the menu, Settings, and click Plugins. Inside the plugin page, click Themes at the top and you will see the different themes that can be installed. I would recommend starting with any-bootstrap-theme, this will give you a lot of options for different bootstrap- based themes. Click install, and you should be taken to a page where you can select a theme from a drop-down and change some parameters. You can come back 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 accessible 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 the Users and security settings menu, go to the Roles tab , and from there you can pick the theme you would like to use for each role