I don't understand what blue button you are talking about.
To clarify some things:
- The volume slider has an indicator (an orange led) that lights if the volume is not at 100%

Near the player controls there is also a led indicator.
It's green when MB is playing, it's orange when paused, and it's off when stopped.

- The progressbar shouldn't have 'a dot'?
Just a bar that changes its brightness.

- Yes, there is a lot of empty space in the player panel.
It's intentional, and why I called it Void ;-)
And as I said before, the small sized icons and controls are intentional, and (on my screen) are perfectly readible, having similar sizes to the text as it shows up in the panels of MusicBee.
- Invisible borders
If you are talking about the vertical handles to drag the width of columns, I fully agree:

Regrettably, skinners don't have much control over it.
I have raised this before on the skinners board (a long time ago), accompanied by a request, but it wasn't rewarded.
Probably because it's less of an issue for light skins and high contrast dark skins.
My skins are often medium to low contrast, and the visibility of that 'dragging indicator' sucks for pretty much all of them.
Sorry for that. I don't like it either.