VirtualBox 5.2 introduced a new change in how it renders the Qt interface. This means that once you install the application, you may find it looks completely out of line with the rest of the system:
There's a way to fix it -- but only in 17.10 and up!
In a terminal:
sudo apt install qt5-style-plugins
sudo nano /etc/environment
Paste the following line at the end of the file:
If you're unfamiliar with the terminal, copy the line above and press CTRL + Shift + V to paste it, then press CTRL + X to save then press Y to confirm.
Log out and log back in to apply the changes.