Print theme summaries in *Custom Themes* buffer.

Print theme summaries in *Custom Themes* buffer.

* lisp/cus-theme.el (describe-theme-1): Use custom-theme-p.
(custom-theme-summary): New function.
(customize-themes): Use it.

* etc/themes/light-blue-theme.el:
* etc/themes/misterioso-theme.el:
* etc/themes/tango-dark-theme.el:
* etc/themes/tango-theme.el:
* etc/themes/tsdh-dark-theme.el:
* etc/themes/tsdh-light-theme.el:
* etc/themes/wheatgrass-theme.el:
* etc/themes/wombat-theme.el: Tweak summaries for better listability.