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

Since 2451b0b99ad8d02e1367827741323d40f870ad91 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
Tests: Ensure custom select dialog background matches desired overlay theme.