Sometimes customers hand out a sheet containing the company colors as RGB or hexodes. Then you can simply copy and paste the colors into the JSON and CSS-files.  If you are less lucky and need an inspiration regarding the coloring of the app, it may help to have a look at the company’s website and consider how they designed it: How did they color their buttons, headers, footers, contents, charts, etc.

If you want to find out, which colors they used, you can use the browser Add-on ColorZilla (https://www.colorzilla.com/) and get the exact hexcodes, which you can then paste into the JSON and CSS files.

The Qlik documentation custom theme properties is located here: https://help.qlik.com/en-US/sense-developer/February2022/Subsystems/Extensions/Content/Sense_Extensions/CustomThemes/custom-themes-properties.html