fix(datatable): simplify default css

Remove the background CSS in the `:dark` pseudo selector, as this is
already the default background for the `DataTable`.

Currently this duplication makes styling a custom widget that extends
the `DataTable` unintuitive (see Textualize#5481).

src/textual/widgets/_data_table.py

@@ -343,7 +343,6 @@ class DataTable(ScrollView, Generic[CellType], can_focus=True):
         }
 
         &:dark {
-            background: $surface;
             & > .datatable--even-row {
                 background: $surface-darken-1 40%;
             }