Hello, 

You can change a theme color using changes in your web module and file path is web/static/src/scss and file name is primary_variables.scss and In this file search // Colors and you get below code.

$o-community-color: #7C7BAD;
$o-enterprise-color: #875A7B;
$o-enterprise-primary-color: #00A09D;

Maybe you get your answer.

Thanks

Video Link: https://youtu.be/amHpruXQMOg

Module Link: https://github.com/hareshkansara/odoo_tutorial/tree/13.0/odoo_theme_backend