I feel stupid, but I have another problem now after updating MusicBee and downloading again your skin.
Now the progress bar is cut and isn't displayed correctly, look here what I get:
No need to doubt yourself. Concerning skins and skinning, both MusicBee and my skins are works in progress for v3.1 at the moment.
I updated the skin to (probably) fix the latest issue you mentioned. Same download link.
What a relief to hear such thing, I though I was abusing your patience, that's why I was feeling stupid and everything, I'm happy to see I'm a pain to read for you.
Your fix works great, concerning me, there's no more problem, so thank you a lot for you consideration about my reports, but since I saw two other things by now I will report mostly for you since they aren't much a big deal for me.
1 - It isn't very important, but there's a slight problem (I think) with the beginning of the progress bar, sorry if it isn't a problem, I'm basing my opinion on your first screenshot where I don't see this, but maybe it's because I'm on a full HD screen or something, I don't know, here's the screenshot.

2 - By updating MusicBee, I saw now you can switch between the progress bar and a wave bar, but the wave bar is green.

Sorry to repeat it, but thanks for being that quick, effective and sympathetic in your answers, I appreciate a lot.