Great work with MP6, it's looking fab!
There is a slight presentation issue with checkbox fields within the "Edit Gallery" media frame interface. The "Show Captions" check box is not rendered correctly.
I have also added custom checkbox fields in this interface which are also not rendering correctly.
Adding the hack "!important" to the width attribute in colors-mp6.css line 95 resolves the issue. It seems that the checkbox is being squashed by width: 65% and then by width: auto.
I hope that this is of help :)