Is it possible to apply spfont, spcolor files to current look & feel without having to visit design gallery - _layouts/15/designgallery.aspx -, or code e.g. Javasript; which changes everything including application & system master-page?
Thoughts:
Currently I can think of creating an entry manually in design list - _catalogs/design/AllItems.aspx - with a name Current, with required masterpage, spfont & spcolor location. I'm looking at more sposphicated way which is more business user friendly.
You could try to use Change the look section itself. Below is a smart way!
You'll be needing below:
You've color palette, font file & master-page. All you need is a preview file. You can copy seattle's preview file & rename as your-master-page-name.preview. This will ensure you've a entry of your master-page in change the look section. Background-image being optional is not a concern.
This way you could request business user to select appropriate master-page, font & color & apply styling without any custom development.
Note: This will change system master-page as well.