I am looking for a lightweight linux desktop to use as an Home Theatre PC, that has UI scaling.
I think you could use Elementary OS for that. It is a lightweight distro based on Ubuntu with a minimalistic UI and HiDPI support. Although, it seems like they are using a workaround for this, according to one of the designers. ("update" paragraph, before the "takeaway" list). Here is another discussion on that subject.
I hope it helps!
You might try Linux Window Managers such as Fluxbox or OpenBox.
If you want to install Fluxbox, first install Ubuntu Mini, then install window manager and login manager:
sudo apt-get install fluxbox xorg slim
Then open console by right clicking desktop: Applications > Shell > Bash
xrandr to see options, type
xrandr -s 1920x1200 to change resolution. Mplayer is a good video player which you can start from console.