Access the source code of the Media Manager and locate web.config. Edit the file and change the following variable.
Change <add key="UseDist" value="true" />
To <add key="UseDist" value="false" />
This will ensure that you can see the live changes until the edit is completed.
Access the source code of the Media Manager and navigate to: MP2 → style_overwrites and locate variables.less. In this file you will make all the changes required to make a custom style.
Original file is located in *site-name*\MP2\src\app\mp2\layout\less.