As a workaround, what worked for me was:
$ sudo apt install systemsettings kde-config-gtk-style kde-config-gtk-style-preview oxygen-icon-theme
* systemsettings for the app systemsettings5;
* kde-config-gtk-style enables the Appearance module in systemsettings5;
* kde-config-gtk-style-preview allows previewing the themes without restarting the GTK applications;
* oxygen-icon-theme is an alternative theme to use in KDE applications.
Then, run systemsettings5, click on Application Style, select Oxygen as a Fallback theme, click on Apply.
As a workaround, what worked for me was:
$ sudo apt install systemsettings kde-config- gtk-style kde-config- gtk-style- preview oxygen-icon-theme
* systemsettings for the app systemsettings5;
* kde-config- gtk-style enables the Appearance module in systemsettings5;
* kde-config- gtk-style- preview allows previewing the themes without restarting the GTK applications;
* oxygen-icon-theme is an alternative theme to use in KDE applications.
Then, run systemsettings5, click on Application Style, select Oxygen as a Fallback theme, click on Apply.