transparency

Hi there! I'm pretty lost.. maybe someone knows what's happening with my imac? I updated to Mojave and since then all my Windows are weirdly seethrough! I've attached a picture of the problem. Ignore the black block on the screen ;) Anyway, in the image you can see photoshop in the front...

I watched a youtube video where the presenter created a new slide using a black background in keynote. Later in the video, he suggested using the slide inspector and change the opacity of the black background from 100% to 0%. I have keynote v8.1 and nowhere could I find the ability to change the...