QCAD Version: 27
Downloaded package (file name): qcad-27.0.0-pro-linux-qt6-x86_64.tar.gz
Operating System: Linux Mint 22.3
If I specify a theme other than Default in V27, then close it, then reopen it the theme is displayed, I then close v27. If I then open v3.33.1 the selected theme is displayed as expected, I then close v3.33.1 and reopen v27, but the Default theme is displayed.
This is expected behavior. When updating from v3.x to 27.x, the theme is reset to Default. The user can then choose to set the theme again if they want. Switching back and forth between 3.x and 27.x is not expected to be a common use case.
You could use different configs for 3.x and 27.x if you need to switch back and forth a lot, e.g.:
For QCAD 3:
./qcad -config /path/to/config/for/qcad_3
For QCAD 27:
./qcad -config /path/to/config/for/qcad_27
You’re right about switching between Qcad versions Andrew. And having different configs would work, I’m assuming I’d use the -config flag on the end of the command line ?
Yes, if you are starting QCAD in a terminal, simply add the -config switch like in the examples above. If you click a desktop icon or start menu, it’s a bit more involved.
I must be doing something wrong, I use this in the terminal to start Qcad…
/home/pete/opt/qcad-3.33.1-pro-linux-qt5.14-x86_64/qcad -config /home/pete/.config/QCAD/QCAD_3
2nd line in the terminal is…
absConfigFile: "/home/pete/.config/QCAD/QCAD_3"
and further down I get…
19:48:09: Debug: openFiles: -config,/home/pete/.config/QCAD/QCAD_3
but Qcad only reads and writes to the QCAD3.conf file ?
You’re right, the -config switch has actually no effect on Linux (only Windows and macOS).
This will be addressed for the next release:
QCAD Bug Tracker — FS#2760: -config switch has no effect on Linux
3.33.1 exhibits the same problem with the -config switch, although that wouldn’t be a problem with v27 fixed
I’ve now downloaded 27.0.2, I run it via the terminal command…
/home/pete/opt/qcad-27.0.2-pro-linux-qt6-x86_64/qcad -config /home/pete/.config/QCAD/QCAD_27
the terminal shows…
absConfigFile: "/home/pete/.config/QCAD/QCAD_27"
.......
.......
08:20:42: Debug: openFiles: -config,/home/pete/.config/QCAD/QCAD_27
…but qcad neither read the QCAD_27 file or the QCAD3.conf file and it didn’t write to either of those files when I closed Qcad down.
Note that /home/pete/.config/QCAD/QCAD_27 is not a file but a directory. The path is created by QCAD and in that directory, a sub directory QCAD with file QCAD3.conf should be created. This seems to work fine here with QCAD 27.0.2 on Linux.
Thanks Andrew, yes, that works fine 
Great, thanks for letting me know.
Just another point Andrew, using the -config flag within a Linux Mint desktop launcher does work without going through the terminal, not looked at Windows though.