This is a technical guide similar to Lumina's.

Useful tutorials:

https://www.youtube.com/watch?v=jaboWUNfMg0
https://www.youtube.com/watch?v=7DBjpMiV17k
https://www.youtube.com/watch?v=3Q1DLqAjtog

Only edit local files when strictly necessary.

If you want to make manual adjustments to the ThemeMixer Key saving process, locate the ThemeMixer.xml file at
C:\Users\AppData\Local\Colossal Order\Cities_Skylines

ThemeMixerSettings.xml also exists and you can edit this file, however be mindful of putting exact values in there else the mod might break.

For issues and more visit my discord support channel.