It seems the version 1.96 is sensitive to the upper and lowercase of letters in the file format. For example: if the file format is “.FChk”, Avogadro will report an error. However, if one changes it to “.fchk”, it works pretty fine.
In the older version, when open a FChk file, the molecular orbitals will be listed at the right side of the window, but it is not shown in the version 1.96. I would suggest to have it back.
The color of the surface (molecular orbitals and electron density) could not be customised.
Hmm. I’ll take a look. What OS are you using?
It’s on the list, but without more coders, it does take time. 
Yes, it can. I specifically added this to 1.96. Here’s the LUMO of benzene:
Click on the “Meshes” line to bring up the configuration options. You can pick the colors and opacity.
I am using MacOS monterey version 12.4.
The error that showed when I tried to open “.FChk” is: Unable to find a suitable file reader for the selected file.
Thanks for the heads-up… this should fix the problem - at least on my Mac:
macOS.dmg
Hopefully you were able to change the colors?