@Georgina_Edwards to my knowledge, text in a text-box cannot be styled further than what the formatting tools within the text-box allow. So you cannot add a background color to it. To achieve that result, you would have to put your text into an HTML block and style it with the CSS part of it.
Its easier to use a HTML/CSS Block instead of a Text Block if you're going to do any type of custom work. Regardless, we'll have to see your site or page that has the Text Block.
Didn't see your response @Lille Ulven until I came back to post a how-to I just made.
@Georgina_Edwards This may help: https://gallery.imagesinthebackcountry.com/Smugmug-customization/Create-a-Colored-Background-to-Text-Using-a-HTMLCSS-Block
@Hikin' Mike no problem , after all, we are both just trying to help out someone else
I am glad you could write up a howto on your blog for it - for now, I stick to my travel stories and the Lightroom howto on mine.