Ublue immutable OS has a read only filesystem and this is some kind of notepad.

cp root_ca.crt /etc/ssl/certs
trust anchor --store /etc/ssl/certs/root_ca.crt

Firefox is running as flatpak and does not honor this trust. Import them in Firefox as well.

/etc/default/grub

GRUB_TIMEOUT=5
GRUB_TIMEOUT_STYLE=menu
GRUB_TERMINAL_OUTPUT="console"
GRUB_DEFAULT="Windows Boot Manager (on /dev/nvme0n1p1)"

Themed grub

/etc/default/grub

GRUB_TIMEOUT=5
GRUB_TIMEOUT_STYLE=menu
GRUB_TERMINAL_OUTPUT="gfxterm"
GRUB_DEFAULT="Windows Boot Manager (on /dev/nvme0n1p1)"
GRUB_THEME="/boot/grub2/themes/asus-tuf/theme.txt"
GRUB_GFXMODE=3840x2160

Then

ujust regenerate-grub