Hi – here is the way I do it.
1. Create an option page – something like “Theme Settings”
2. Add Fields for each css style I need
3. Group them in tabs for for mobile/tablet/desktop/HD
4. Define a hook which is fired once the option page is updated
5. The hook is generating a css file / it updates the css file
6. include the css file in your theme in function.php
If you want to go more advanced. Create a vars.less (I am using less and uikit) and compile the css with a less compiler, after the option page is saved. Like this I only load one css file
