DKG Theme Modifier - For use with Themes by DavidKGriggs

[waah]

I am unable to install DKG Theme Modifier on Playnite 9. I downloaded v 2.4.1 on Github and ran it - Playnite asks to restart, then nothing happens. 

I've also attempted to install PlayniteModernUI v 1.3.1, but that's not showing up under "Themes," so I figured I needed the DKG Theme Modifier.

I run into issues trying to follow the installation directions, as my copy of Playnite does not have "Plugins":

In Desktop Mode go to Settings, Plugins, DKG Theme Modifier like the screenshot below, you can then choose to check for Modifier updates, you will then see tabs for each theme you have installed:

Did I miss a step somewhere?

You didn't miss a step, there's been a change. I was having the same issue. The Extensions-tab gives a hint. 
It's been moved to Add-Ons.

Go to Add-Ons (F9)
Extension Settings
  Generic
    DKG Theme Modifier

The rest is the same it seems. It just moved from Settings to Add-Ons.