Originally Posted by Doitsu
You could create a custom clip called "Yellow" with the following data:
Text: <span style="background-color:#FFFF66;">\1</span>
Then simply switch to code view, select Tools > Clips to open the Clips window select the Yellow entry and move the Clips window to the side.
Each time you find something you want to hightlight, click the Paste Clip button. This will apply a yellow background color style to the text that you selected, which is of course only visible in Book View mode.
Actually, to use a clip, just highlight the text then right click on it and you'll see the clips in the context menu. The next version will actually provide a dockable Clips window like the Table of Contents window to make this even easier.
Although I don't think this will help the OP much as they want to do this in Book View. All the talk of clips and styles is probably a bit further down the line. But if they really want to go that route they would do well to read through the user guide regarding styles, and then clips.
For the OP, one of the reasons that Sigil doesn't have a Book View button to change the foreground or background colour of the text is first because no one has asked for it. Mostly because most books are not in colour, and coloured text may not show up well on most of the greyscale readers. And not every action that someone wants can be made a button - there are just too many unique cases and most of Book View's buttons are just meant to be a basic way to style text. What everyone is pointing out is that you can create your own 'button'-like menu item (a "clip") that pastes the code necessary in Code View to make the text a certain colour. But as you said - that's in Code View and may be more than you want. Remember that Sigil is for creating ebooks as everyone has said, and its not really a reader with highlighting abilities.