I want a vanilla-PC that when turned on highlights the first menu entry (plain "Ubuntu") and then starts without any frame.
Your screenshot in post #1 shows you have a grub theme in use. The frame with the wide border is part of the theme. If you disable the theme, you will have the default grub menu.
The theme is usually specified in the /etc/default/grub file with a line like this:
Code:
GRUB_THEME="/usr/share/grub/themes/ubuntu-custom/theme.txt"
Add a # at the beginning of the line to disable the theme:
Code:
#GRUB_THEME="/usr/share/grub/themes/ubuntu-custom/theme.txt"
After this, run sudo update-grub to rebuild the grub.cfg file.