I have put those lines into
update-grub was done after that)
However, GRUB doesn't look right - it has black margins around, renders noticably slow (takes about 1-2 seconds to fill the screen), and the font looks weird - it feels like it makes 1920x1080 image but tries to fit it into a smaller resolution.
After installing Nvidia proprietary drivers, the problem also applies to plymouth.
If I type
vbeinfo in GRUB's console, it does list 1920x1080 with 32-bit color mode.
What can I try to fix it?