Hi @einsfeldleo
It appears this is a defect that has been documented at https://www.ptc.com/en/support/article?n=CS316985 .
If the value is not bound, then the style applied to string > value-container works, but if the value is bound, the default style remains.
As a workaround you can define your own Style Theme, set the color for Colors > Text Colors > Body
Then apply the style Theme to the value display in the mashup.

Then the value dispaly text follow the color define in style theme:
value display text follow the style theme
Hope this helps
Christophe