OOo Writer has different categories of styles, so the first step is to distinguish between them. Here's an overview over the different categories (from OOo docs):
Make sure you're working with Paragraph Styles if you want to format cells: The cell formatting depends on the Paragraph style of its content
If you've created a new style as you've described, make sure that it has the desired style definitions. You can access (and modify) them using the "
Modify..." entry from the style's context menu (again, OOo docs):
using the paragraph style's border and background settings, you can modify table cells, too. If it doesn't work for you:
- check if the paragraph style has the proper settings;
- reset the cell style to the paragraph style defaults using Menu "Format" -> "Default Formatting";
- Select the cell or its text and apply the paragraph style again.
This works at least in OOo 3.2 and LibreOffice 18.104.22.168.