"Provide a custom form" is absolutely the wrong way to do it -- that will remove the default forms off the page and you're just making far too much work for yourself!
An easier way would be to use CSS to override the default of "Custom Skin 1" or "Galleria Skin."
CPGuest > Admin > Plugin Manager > Custom Skin 1 > Config > Mode: Advanced && CSS :: Advanced CSS :: exclude whatever you don't like (e.g. background colors and borders)
... And then add whatever styles you want in the CSS styles box, e.g.:
{literal}
.nwaTop {
background-color: pink;
}
{/literal}
Or stick to Galleria:
CPGuest > Admin > Plugin Manager > Galleria Skin > Config > (paste your CSS overrides into HTML HEAD)