That's not a Sweet Home 3D problem. It's the desktop environment you are using on your (old) Ubuntu distribution (Gnome?).
Sweet Home 3D for Linux adapts to the installed desktop so it will look a little different on Gnome, KDE, Mate, Cinnamon, XFCE, etc.
Try another desktop and it will look different. First look up the different desktops to see what you like, choose one of the big ones to be safe. And first update your Ubuntu
https://tecadmin.net/how-to-install-and-switch-desktop-environments-in-ubuntu/
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