I know there is a "game.text_shadow_color", but can you change the opacity?
Not with the built-in functions, but I have some code that can easily be modified to do it (and which I have in fact modified to do so, in unofficial versions of the SpeechBubble module):
https://www.adventuregamestudio.co.uk/forums/index.php?topic=55544.msg636576524#msg636576524
Sorry, didn't read your answer till now. Funnny enough, I'm already using your amazing SpeechBubble module in my game. Thank you very much!