I just tried the captions and they look good so far bot how do you control the background colour and font colour?
You can set the caption colors and fonts from the "Automatic Captions" options tab, but this is likely to change and i'll probably put most of those controls in the Caption Dialog itself.
However, note that a caption is an EDITABLE text box object, so once it's created you can select it, and change the text,colors, font, and transparency.
My plan is to add a feature where you can save and then choose presets of option combos for these text boxes and captions.. so you could easily choose "Large white text on semi transparent background at the bottom" vs "Small Red text overlaid on image at top with no background" etc., and same for text boxes.