You can customize and add theme to Dropfiles, but we don't use the standard MVC Overrides but the Joomla default plugin system.
Droppics themes are in fact plugins and they are located in the folder /plugins/dropfilesthemes
You'll have to copy one plugin, change the name of the plugin to create your new plugin (pluginname.xml and the class name of the plugin). Then install it through Joomla standard installer, it should be shown in the list of themes. You can then do all changes you want in this theme, it will not be affected by updates.
Dropfiles is built to be used in every WYSIWYG editor field, you just need to have content plugin to be called. Most of the time it's activated by default.
It's not the case in the Joomla HTML custom module, you need to activate it.
In the option tab you need to set the option Prepare content to Yes.
Yes, you can add a single category (without any subcategories) or categories that contains subcategories.
In the second case, subcategories will be listed and you can navigate through them.
Dropfiles allows you to add single file or categories of files directly in your WYSIWYG editor.
Dropfiles is compatible with all editors, it means that you can add files in an article, a custom module or in any component that calls content plugins.