Well the simplest way is just to right click it -> settings -> transparency -> X%
Mine is edited,
;Text colors.
TitleFontColor=255,255,255,160
ItemColor=255,255,255,160
PriceColor=255,255,255,80
Ctrl+F ;Text colors
the colours are in format RGBA, so just modifying the last integer is all you need to add semi-transparency.