In Ubuntu, /etc/grub.d/30_os-prober overrides grub menu style and timeout defined by the user in /etc/default/grub. Here is a workaround to save the menu style and timeout values before os-prober changes them, and to restore them afterwards. The two files need to be placed under /etc/grub.d and made executable. Finally, "sudo update-grub" should be issued in the terminal.

## 25_pre-os-prober
#! /bin/sh
set -e

# Save the $timeout and $timeout_style values set by /etc/grub.d/00_header
# before /etc/grub.d/30_os-prober messes them up.

cat << EOF
set timeout_bak=\${timeout}
set timeout_style_bak=\${timeout_style}
EOF

## 35_post-os-prober
#! /bin/sh
set -e

# Reset $timeout and $timeout_style to their original values
# set by /etc/grub.d/00_header before /etc/grub.d/30_os-prober messed them up.

cat << EOF
set timeout=\${timeout_bak}
set timeout_style=\${timeout_style_bak}
EOF