[FIX] web_editor: properly reset to custom background option

This commit solves a bug when using custom background option: if a
custom background was selected, then hovering a non-custom background
and leaving the dropdown was selecting the non-custom background for
good instead of resetting back to the custom one. This is because the
custom background option is a non-preview one (you have to click) so
there should not be any need of resetting... except that the non-custom
background option is a preview one and messes with the same DOM
attributes as the custom background option.