I’ve configured my PC to use the ubuntu-mate theme, when starting PC, via the Grub Customizer.
After upgrading to Ubuntu MATE 18.10:
- starting PC shows a grey / black screen
- no possibility to press ENTER in order to speed up the boot process
Solution for my PC:
- start Grub Customizer
- switch to Appearance settings (see image below)
- click advanced settings
- double-click the GRUB_TIMEOUT_STYLE setting
- type menu
- click somewhere outside of the field
- click Close
- click Save in the Grub Customizer window
- restart PC
Result: Grub will display the set-up theme.
Hope this helps.