shopify website update

I am looking for a Shopify expert to help me with functionality improvements for my website.

Specific updates needed:
- Implementing specific improvements that I have in mind like
a. Website currently support DEFAULT Theme. Need to implement international theme.
b. File upload needs to be implemented
c. small design changes

Additional requirements:
- Ensuring that the updates are mobile-responsive

Ideal skills and experience:
- Strong expertise in Shopify development
- Experience with implementing functionality improvements
- Knowledge of mobile-responsive design principles