I use w3 total cache for caching, minifying and concatenating stylesheets. I find that all optimization plugins require a little patience and configuration to work well. With regards to working well with Microthemer, just remember to use the ‘clear all caches’ option in w3 total cache after making some changes in Microthemer. Otherwise your style changes will appear for you (as a logged in user) but not for your site visitors (who will see cached pages).
I hope that helps, if you need further assistance please let me know.