Thanks Diap Dealer and theducks. My newbie head starts to hurt a bit...
Well, I didn't color anything. I did exactly as was laid out in a tutorial (how to make an e-book with Sigil) in a Dutch computer magazine,
ComputerTotaal.
Then in Word 2013 I choose the font Calibri and adjusted the font size for title and text.
I put a few words and the title in
bold and I added a few links.* That's all I did. After that I saved the file as 'Webpage, filtered' and I opened the document in Sigil.
So I don't understand what went wrong? And I know nothing about styles.
* In Word 2013 the links look blue, could that be the problem? And if so, how can I adjust the linkcolour to whatever colour is allowed?