there is no transparency for bottom text messages. Every text there is always on black background.
Of course you can change font, size and color of those messages. In "Project Properties" choose "Plugins" and check "Change Message Font". You can edit it by pressing "Configure the selected plugin".
This plugin doesn't work with dialogues. In that case in frame where the dialogue is, you must open "Frame Properties", go to "Advanced", check "Execute VBScript..." and place there those lines:
For Each X in DialogueObject
X.FontName = "name of chosen font"
X.FontSize = 16
X.FontUnderline = False
X.FontBold = False
Luminous beings are we, not this crude matter...