I think that this archive should contain everything you need to run mGalaxy v2.1!
www.mgalaxy.com/downloads/mGalaxy_v2.1.zip

Oh yes! Sorry, my mistake! I sent you a file that is already updated for the next version of mGalaxy!
Here is a file that should work better!

Call the 'Preferences' panel (default key to call it is 'Shift')
There you will find sliders to set the Music and Video volume when the screensaver is active!

You have to SCROLL the dropdown menu to see the other themes

What is listed in the dropdown menu? May you please send a picture of it?

Did you run a test with another emulator?

Could you do the test with another emulator and tell me if the same thing happens??

Also when the mGalaxy-Window is minimized during change the games, Mame disappears.

I don't understand your sentence very well, could you clarify this ?

Can you check if, once quit, MAME does not appear anymore in the task manager?
You can get access to the Task Manager by pressing CTRL + SHIFT + ESC at the same time

Enclosed you will find 2 files.
Copy 'Emulators.xml' into mGalaxy > Data > Definition folder
Copy 'BGB.au3' into mGalaxy > Data > Scripts folder

...and voila

I'm pretty sure it's actually MAME that's not really quitting and mGalaxy is simply waiting for it to exit!
To test it, place the attached script (MAME.au3, just below) in mGalaxy>Data>Scripts.
This script will have the effect of "killing" MAME.

Then come back here to tell me if it worked!

You're absolutely right...I added it recently and you can enjoy it in the next version!

No, this is not the case at the moment, sorry

From what you describe, it is possible that your version of Mame is so old that it doesn't match the catver.ini, history.xml AND history.dat (if you can find it), nplayers.ini...files that are used (and automatically downloaded) by mGalaxy to build the database.
First, I suggest you to delete these ...

You know that you have to drop the bios in 'rom' folder of Mame, do you? (Read the 'Requirement' info for Mame in the 'Emulators' tab!)
...the same is true for Retroarch..but it need to find the bios in its 'system' folder!

Take the 'Epoch Super Cassette Vision' system for instance.
Retroarch need ...

1. In mGalaxy > Data > Definition, create 4 folders named
Epoch Super Cassette Vision
Tomy Tutor
Bit Corporation Gamate
Game Park GP32
2. Put the corresponding (joined) icons in each folder (they must be named 'icon.png')

3. Open mGalaxy > Data > Definition > Systems.xml, add those systems ...