Hey there, @KA1
Bo here from Shopify Support!
That is a really great question. While it is possible to customize your them from the Shopify App it can be rather difficult as you cannot see the live changes in action, and for this reason, we would recommend altering your theme from a desktop primarily. I would recommend taking a look at this guide on customizing Shopify themes.
If you are looking to alter the theme from your phone you can go to Store > Online Store > Manage Themes > Customize. From here you can add and remove sections but you cannot see these live changes.
How are you liking the platform so far? I'd recommend taking a look at our Shopify webinars while you are getting started. These short tutorials takes you through all the basics of creating a store such as adding products, variants, collections, theme changes and pointing a domain to your store. You should be able to sign up here if you would like. We also have a free business course with our Shopify Academy that I would really encourage you to look at if you get a chance.
All the Best,
|an hour ago|