Hello
If you don't want to use the default presets, then yes you can use custom presets. You can change the maximum color & background with other property. Besides, If you need more modification, just create a custom.css file inside your templates/your_template/css/here & write down your more complex CSS on this file.
Or if you want to change preset & open the option.xml file and change the color code
templates/indigo/options.xml
https://prnt.sc/omhx0x
After changes in some CSS code from template settings, please do clean your Joomla cache and make a hard reload.