I have an older project I brought into Windows Version: 3.1.1.0 (1463331) 64-bit - 03 Nov 2020. When I change the theme, it never updates the text color. So, In the case of a dark theme, I get a black text on a dark background.
If the Editor text is set to No Color (upper left square with diagonal line, as shown below) then it should adopt the text color as per the theme.
However, if at some point you changed the text to a specific color, in this case probably black, then Scrivener assumes you want to override the Theme color.
So try this as a test -
Select some text in a document, right-click and select Remove Color. This will change the text color to No Color.
The remove color works, but I haven’t found a way to change it for all documents. Going through three novels section by section is really really painful. Any way to change it all at one?
First, set the default text color for new documents to No Color. File > Options > Editing > Formatting.
Then you can use the Documents > Convert > Text to Default Formatting command to change multiple documents at a time. See this Knowledge Base article for details. In the article they call it " Normalising Formatting in Existing Text".
Be sure to check the Convert Font Only box if you don’t want other aspects of formatting to be impacted. If you’re using multiple fonts in a particular document, then you probably don’t want to use this method with that doc.