Making some CSS changes to awebsite?

I am needing to make some changes to this website for a client but can’t figure out how to do them:

-I need to change the donate link in the navigation to a button that’s distinguishable. At this point, I have added a class to the navigation item. The class has been named myButton and I have applied styling but nothing is happening.

-Also, the white background where the image is located need to be transparent; however, I can’t make that happen.