For the vast majority of games which are using the "default" renpy user interface definitions, it's actually quite easy to change the transparency of the dialogue box background manually:
1. Open the "game/screens.rpy" file in a text editor (even windows notepad if you don't have anything better)
1a. OPTIONAL: If the "screens.rpy" file is not present, use the "Unren for all" tool to extract the rpy files (
https://f95zone.to/threads/unrengui-unren-forall-v9-4-unren-powershell-forall-v9-4-unren-old.92717/)
2. find the
screen say(who, what):
section
3. add the line starting with "background" into the code
Code:
screen say(who, what):
style_prefix "say"
window:
id "window"
# add this line in this position _ _ _ _ _ _ _ _ _ _vvvv------ Edit this to change the transparency
background Transform(style.window.background, alpha=0.75, xalign=0.5, yalign=1.0)
if who is not None:
window:
id "namebox"
style "namebox"
text who id "who"
text what id "what"
NOTE!!: Pay attention to the indentation of the line! It must match the line above! Use spaces, not tabs.
4. Save the screens.rpy file, restart the game, check if the transparency is what you want.
5. You can adjust the transparency level by manually editing the file again and changing the value of the "alpha=0.75" to other values between 0.0 (most transparent) and 1.0 (least transparent)