I know this post’s a bit old, but I found a solution to the same issue I had. I wanted to have a custom color for text boxes.
When editing the text content of a text box, click on the “text” tab to switch from “Visual” tab. Then insert a small html box that create a cell with a custom color in background. Then, write your text into that cell.
<table><tr></td bgcolor=”#FFFFFF” width=”100px” height=”100px”>YOUR TEXT CONTENT HERE</td></tr></table>
I don’t know your skills so let me explain:
#FFFFFF is the color code for the background color
width=”100px”: Width in pixels of the cell you created.
height=”100px”: Height in pixels of the cell you created.
In the choice of your theme color provided with the theme, select “none”. Then you’ll just have to figure the size of the text box by playing with the “width” and “height” value in the html code.