Regarding CinGG theme, I have wondered what cause legacy desktop keep legacy CinGG program window with grey left and top panels and some red buttons,

It's visually better to exemplify this with images:

In the previous CinGG Quickstart, Figure 2 and Figure 7
https://cinelerra-gg.org/download/CinelerraGG_Quickstart.pdf

which except for the FFmeg button looks similar as Cinelerra-cv
http://cinelerra-cv.wikidot.com/cincv-manual-en:program-window

while newer installations get sharper black and white and green buttons?

as in the current CinGG manual
https://cinelerra-gg.org/download/CinelerraGG_Manual/Video_Audio_Tracks_Navigati.html#fig:patchbay

The differences is found in Settings > Preferences > Appearance : Layout
So my own idea is that when installing newer CinGG versions, existing legacy layout settings won't be overwritten.