I am still new in running desktop environments on xRDP. When I first installed XFCE and applied arc-dark theme, it worked fine and similar to the screenshot shown here.
Then I decided to change to KDE Plasma. So, I purged kali-desktop-xfce by using
apt purge kali-desktop-xfce apt install kde-plasma-desktop echo "startkde" > ~/.xsession
But, KDE Plasma was lagging. So, I decided to purge KDE Plasma and switch back to XFCE.
apt purge kde-plasma-desktop apt install kali-desktop-xfce echo "xfce4-session" > ~/.xsession
To my surprise, the configuration that I did was still there when I ran XFCE again, and that includes the arc-dark theme being applied. But, I faced a new problem that I never faced previously. The theme is broken, and I've spent hours searching for solutions, but found none that had something to do with what I am facing in the screenshot.
I tried changing to other dark themes, but the white parts wouldn't change according to the themes. The top panel is also broken as in the font is dark color instead of white (or any other light colors). I suspected that this is happening because of GTK, but still couldn't figure out the real issue.