Lines only are well defined if hard line breaks are used. The actual subdvision of a text is by paragraphs. For paragraphs you can use the format dialogue and choose the tab ‘Area’ (formerly ‘Background’). There you are offered fill styles. Choosing ‘Colour’ will open a dropdown for selection.
If you insist on setting something like a background colour for a line instead of a paragraph, you have to use a workaround:
- Set a ‘Type’ > ‘Right’ tabulator to the right end of you “line” (will afflict the complete paragraph, of course).
- Insert a ‘Tab’ behind the last printable character of your line.
- Select the complete line (including the tab mark) and set your colour as highlight colour, which is a character property.