Hi @Maria_la_Portuguesa, thanks for getting in touch. That does look strange indeed, it could be some extra formatting pasted in from some other rich text application, did you paste that in from the web or from microsoft word kind of app?
It would help to have a link to the published url of a page showing the text with the black background.
Also, a read-only link to the site is really helpful
If you do not want to share this information publicly, send a message to the support desk: Webflow Customer Support | Webflow University with all the info.
On the issue with the condition where the fonts showing the fallback font briefly, is a condition known as FOUT, a flash of unstyled text, which is a common google font issue.
You can read more about the issue, which has a CSS workaround: Flash of unstyled text - fallback fonts loads initially then switches to google font - #7 by cyberdave
Here is how to add custom code to your project: http://help.webflow.com/faq/how-to-add-custom-head-and-body-code-to-a-webflow-site
I hope this helps on the font issue. This is still open while a permanent solution is checked into.