Applies to ComboBox. Two properties are about the text an editable combo box accepts — a ComboBox has no MaxLength or CharacterCasing of its own, so these fill the gap that TextBox does not have. Three more are about the list that drops out of it.

<ComboBox IsEditable="True"
          mah:ComboBoxHelper.CharacterCasing="Upper"
          mah:ComboBoxHelper.MaxLength="10" />

Both only apply while IsEditable is True. On a drop-down-only combo box there is no text box for them to reach.

The drop-down

On develop the fill, the frame and the corners of the list are the box's own rather than three keys everything else reads too. A list hangs over whatever is behind it, so it needs a chrome that can differ from the one the box wears, and a set that gives its boxes a look of its own wants the same one here:

MahApps.Styles.ComboBox sets the first two from MahApps.Brushes.ComboBox.PopupBackground and MahApps.Brushes.ComboBox.PopupBorder, so overriding either key in a theme still does what it always did.

<ComboBox mah:ComboBoxHelper.DropDownBorderBrush="{DynamicResource MahApps.Brushes.Accent}"
          mah:ComboBoxHelper.DropDownCornerRadius="8" />

Related

Most of what else a ComboBox offers comes from other helpers: the watermark and the clear button from TextBoxHelper, the corner radius and focus brushes from ControlsHelper, and the brushes for the items in the drop-down from ItemHelper.