How to install Unity tweak tool and themes in Ubuntu 20.04?

I'm using Ubuntu 20.04 Desktop, and I'm still using apps that do not work in gnome (my VPN for example.)

I've installed Unity 7.5 as my window manager, and everything works, but the theme is not my favorite I switched it in settings->appearance->theme to high contrast, and now I'm stuck with that - there are only two options in the drop-down and both are high contrast it seems.

After selecting adwaita, I think I'm in high contrast mode because my settings panel looks like this:

How can I add a theme?

I followed numerous tutorials that say, install unity-tweak-tool

sudo apt install unity-tweak-tool

It installs but does not run, instead, I see:

$ unity-tweak-tool
Error: schema com.canonical.Unity.ApplicationsLens not installed

It also shows an error in the GUI, but does not launch. I think every tutorial I found to add a theme requires the tweak tool.

Solution 1:

A subset of nycnik's answer got it working for me:

sudo apt install unity-lens-applications
sudo apt install unity-lens-files