Currently, the styles for "wikitable" class does not specify a background color.
Someone might customize their wiki by changing the background color. In current behavior, it might make tables harder to distinguish since the table background color is inherited from page background color.
An extreme example as following:
Obviously, the 2 normal table cells are hard to distinguish its contents.
Then, by assign a default background color to "wikitable" class, it is more distinguishable, as following:
The new table background will use the same color (the variable --background-color-base) as the page background, which will not change the look in default installations.

