When I make a change on a device it also happens on another device, but I can’t change it on the other device and the “automatic” change happens. I don’t know if it’s a microthemer error or I’m doing something wrong because I’m a layman with HTML and CSS.
When you set a style on e.g. a default media query called “< 1200" that applies to all screen sizes below 1200 pixels. That’s because the media query uses max-width, meaning any screen up to the max width of 1200 pixels. So that part is normal behaviour.
It should however be possible to override your “< 1200" style on e.g. the "< 768" responsive tab at the top of the Microthemer interface. That's because those styles use the same CSS selector, but come after the "< 1200" tab, in terms of the order in the CSS stylesheet.
So responsive tabs to the right override tabs to the left. With that in mind perhaps you can organise your styles differently? If you're still confused however, perhaps you could send me a link to the page you're working on with a description of what is happening vs what you want to happen. I can then advise you further.