Custom select: Transfer value of "overlayTheme" option to dialog 

Since 2451b0b both the "overlayTheme" option
and the "theme" option are transferred to the dialog's data-theme attribute,
resulting in two copies of the data-theme attribute being set on the dialog div.

If the "overlayTheme" option is specified, the "theme" option is fanned out to
both the dialog's "theme" option as well as its "overlayTheme" option.

Fixes gh-6975

Edit: Changed title and description to reflect discussion