Regarding forms for comment_form and contact_form. These plugins support custom forms in two ways that will be used if present in this order:
- `/themes/<yourtheme>/comment_form/comment_form.php` -> used for this theme
- `/plugins/comment_form/comment_form.php` -> for all themes then
- standard plugin form is used
Naming of the form file must be as on the original plugin (mention it because the file is just form.php for the contact_form for actually unknown reasons ;-)). I thought we had this documented somewhere… TinyMCE does support custom config files in a similar way as well.
You cannot do this with plugins like the related items. You have to make your own plugin and place it in `/plugins` (note that core extensions use different paths so that might need to be changed depending on the plugin) or place the modified function, if it is just one, within the theme's `functions.php`.