In your second screenshot, notice on the right where it says Type:
there is a grayed out dropdown selection box and a button with a plus sign next to it. Press the button with the pluss sign and you should get a add item type
dialog showing up.
In that box you will want to pick your control node type you want to override with the theme. Pick the RichTextLabel
node class from the list that should show up.
Once you've done that the panel on the right should become populated with properties that can be overridden. From there you want to press the plus sign next to font_color_shadow
then switch to the second tab going from left to right. And in that panel you want to press the plus sign next to shadow as outline
.
Though how well that will work for your font I do not know. It looks god awful on the default font IMO.