I'm about to start a solution that needs quite an amount of layout forms and I would like to create a consistent design based on my own custom theme that has already been developed in a separate file. I use Material Design specs as style guideline and the solution is build under the separation model, so all the data is in a hosted file.

I'm considering one of these two strategies:

• Create a repository of interface elements (text fields, tab controls, combos, portals, etc.) in a separate file (that's mostly done) and copy generic elements into my solution as I build the needed layouts  or
• Create layouts quickly, overlooking style and apply my theme when all of them are functional

I see a drawback on the first option: if I want my element repository to continue useful I will need to keep the theme updates simultaneously in both files

The second option looks more practical because it will allow me to focus on the data and just worry about the look at the end of the process.

What do you think is the most productive strategy?

Option 1.  Updating an element should be infrequent and not too difficult once you get rolling.  Actually your may be better off creating and modifying the elements and objects in your solution, then copying them to your developers reference database.  They're are many template dbs available for rapid, repetitive developement.

When I updated and rebuilt my business DB, I spent the first large chunk of time creating one layout and custom theme, duplicating it, and saving it as the template with all the navigation buttons bars, as the basis for all new layouts.  I also have one template layout for all printing/reports.

Option 2 will kill you in the end.  You'll be going back to all (100's?) of layouts tweaking spacing, and nudging objects throughout the solution.