Theme Responsiveness

Good Morning,
I was doing some clean-up on my site to improve speed and had a plugin crash (not Hummingbird). I removed some old plugins and possibly some that my theme needs. I use the same theme on and The PRomo site is responsive on smartphones, is not.
Can you help determine a solution? Do I re-install the theme...I also have some questions about CSS that relate to Hummingbird and site speed. Google Page Insights shows a few CSS errors...I have no idea what CSS is, but if you can write jokes with it then I will try!
Would you be able to help look at either? Thanks!