Install
This is the repository archive. It installs, but the folder name will carry the branch suffix and updates will not flow.
Install straight from GitHub with WP-CLI:
wp plugin install https://github.com/seothemes/responsive-css/archive/refs/heads/master.zipThis little plugin adds a new section to the Customizer which allows you to add CSS at different breakpoints without having to manually write media queries.
It creates the following custom CSS fields:
- All screen sizes
- Mobile only
- Desktop only
You can also change the breakpoint at which the mobile and desktop media queries change.
Screenshot

FAQ
Will this plugin be added to WordPress.org?
Yes, at some stage it will.
What is dynamic CSS?
By default, WordPress' Additional CSS is output as an inline style in the <head> section of your site. This isn't great as it can't be cached. Instead of doing that, this plugin uses dynamic CSS which is treated just like a CSS file.
Can you add feature X?
Probably, create a new issue here and I'll consider adding your feature request.