When I run the application in the IDE, the window of the screen settings does not contain the resolution below 1024x768. Can I customize the default settings 800x600 without changing the application code?
You can’t, the list is coming from the graphics subsystem and contains all configurations your graphics driver reports as being compatible.
Thanks for the quick reply. This resolution was before, but not now. Probably something in the system was change.
Thank you!
Most graphic drivers are a bit strange in this,
for example mine reports no refresh rate at all in fullscreen 
I solved my problem. If anyone is interested, I set the screen resolution to Auto in the video card settings (nVidia ubuntu). There was a native resolution of 1366x768. Then rebooted. Now the list includes 800x600 and even 640x480.