The MainConfig.cfg file is stored at the root of your theme directory. It is the main configuration file of your theme, and stores identifying information about your theme. It uses the following structure:

<?xml version="1.0"?>
<Theme>
<ParentId>theme.prototype.main.v1.0</ParentId>
<Title>Clouds Orange</Title>
<ShortTitle>Orange</ShortTitle>
</Theme>

Once you enable theme overrides, you can modify the following parameters:

ParentId
ID of the parent theme (e.g. theme.prototype.nature.v2.0).

Title
The full title of the theme including theme set name (e.g. Nature Serenity).

ShortTitle
The name of the theme (e.g. Serenity)

Do not modify this file if you are extending an existing theme, rather than creating a new one.