Site dropping a custom CSS style?

I’ve been trying to get a blend modes to work on the floating hamburger for this site:

And here’s the published site so you can see that the blend mode doesn’t work:

The multiply worked just fine at first but the problem started when I tried to switch from ‘multiply’ to ‘difference’. Only multiply would work. Then I tried messing with the class names and now no blend modes work.

I think the CSS file need to be cleaned up or something. I’m at a loss.