Deleted comment, EbonJaegar is correct in that it's blacklisted for a reason, let's not circumvent it shall we?
The theme labeled "Default" isn't actually default and it is provided by the GTK3 toolkit itself for styling. Makes sense that'd it'd look the same because Adwaita is developed by GNOME and they developer their apps and experience around it.
The actual default theme in Solus 4 is Plata-Noir.