Thank you for your response, Bada!
When I was saying "field" I meant some generic "data cell" element where some piece of information can be stored, whether it is post meta, user meta, or theme mod. Types calls "post meta" "Post Fields", "user meta" "User Fields", so I was trying to extrapolate the same terminology to "theme mods", but maybe it was misleading.
Let me explain what I would like to have.
If you want to attach some piece of information to a post, you can do it with post fields (or post meta in WP terminology). Then each post from a group would have a filed shown in a post editor where you can store some data. That data is unique to the post. It works similar for user fields.
Now let's say I want to store some information that is related to the site as a whole, not to any particular post. Let's say it's a color of a banner that is shown on every page of the site. And I would like to use this color in Layouts and Content Templates the same way I use Post Fields.
WP recommends to use "theme mods" for storing such information. Unfortunately, there is no way, I believe, to create a theme mod within a Toolset using its excellent graphical UI. There is also no way to insert a theme mod value into a template, except for creating a custom shortcode for that.
So I was suggesting to Toolset to create a new type of "field" (as such items are called "fields" by Toolset), along with existing Post Fields, Meta Fields, and User Fields. When a new element of that field is defined (via "Add new field"), it will appear in Theme Customizer. I would also like fields of this type to be available in Custom Template editor the same way Post Fields are so I would be able to insert their value with one click. Internally it would use "set_theme_mode" and "get_theme_mode" to store and retrieve data, but that would be hidden behind Toolset's UI.
Did I describe it clear enough? Ready to explain it more if needed.
Thanks!