Under the heading "Memophenals" I have collected some gists about working with Joplin. Until now these relate to the graphic design, i.e. Markdown, CSS and HTML.
As far as backgrounds of table components are concerned, the visual organization of a table is rather straightforward. Each component lives, dependent on its type, in one of the layers of a stack as shown below.
The direction of view is top-down, so the backgrounds of lower layers are only visible as far as the components above it have a transparent background (which is the default).
Tables defined by (Multi)Markdown code don't contain column or column group elements. That makes the model even more simple.
Borders of different table components may overlap. You can choose to keep them separate by setting the table property border-collapse: separate; so that thay are all clearly visible. The alternative is border-collapse: collapse;, which will make only one of each overlapping borders visible. There are some rules to decide which border, but unfortunately they can be interpreted in different ways.
border-style: hidden; dominate all other border styles.none always lose.