This is kind of old, but this is how I did it:

For new text:

• Set the secondary colour to yellow/whatever with the amount of opacity you'd like.
• At the bar at the top choose Text Style: Fill Background

For a text which is part of the image already (screenshots etc.):

• Add a new layer
• Choose the rectangle tool
• At the bar at the top choose Fill Style: Fill Shape
• Choose the yellow/other colour and draw the rectangle around the text
• When you're done - double click the layer and choose the right amount of opacity for the layer

You can also choose the opacity at the rectangle but that means that

• Once you're done you can't change the level of opacity
• If two if your rectangles are overlapping that means that the opacity of the overlap will be higher than the rest of the rectangles.