Hi!
I had some problems with installing Virtualbox guest additions, so I found this page:
https://sparkylinux.org/wiki/doku.php/virtualbox_fullscreen_problem_installing_guest_additions_on_sparkylinux
What also could be included in this text is:
Before installing the Linux Guest Additions
1. In Virtualbox Settings > Display, change "Graphics Controller" to "VBoxSVGA" and enable 3D Acceleration.
2. Install Linux headers with command
sudo apt-get install linux-headers-$(uname -r)
I tried to change the wiki, but could not log in.
Updated, thank's a lot.