Well that is odd! The theme as shipped should have set the main editor text settings to override the text colour (with desaturation), to Solarized Base1, or #93a1a1. That is I what I use at any rate.
If you haven’t been there before, it is in the Appearance: Main Editor: Colors tab in Preferences. The “Text” setting is where you can globally override without using formatting.
And once you’ve done that, you might want to use the Manage button in the lower left corner to save the theme (you can overwrite the original if you want).