A lot of people don't actually use the Text Module do they?
It's just as good as the data, trigger and terrain ones.
Anyway, You can scroll around in the text editor ( make sure "Data Source" is on "All Data" )
You get a list of all text in the game, all have their own little preview, and the Style Name (which is greyed out in the preview window)
Anyone you can use those stiles names as strings for the trigger "Set <label_name> style to <put your styled here> for (All Players).
Simple? yes, just remember: there are 5 modules in the editor, most people only use the 3 (data / trigger / terrain), and sometimes the Import editor.
Text one is just as important.
Edit: I just noticed almost all the text styles in the text module appear as ErrorDisplay, which is strange as it wasn't always like this.
ErrorDisplay still is a string you can use.
Edit: a good workaround is to make a trigger action "display text message" and in the text window, you can scroll and see all of the styles available.
I recommend "LoadingScreenTitleMarSara"