My theme does not show the 'all apps' icon on the Home Screen. How can I access the rest?

Short answer, either:

• Install random apps to meet the minimum amount of 13, or
• Install a different theme that uses a different layout

Long answer: The theme you installed uses a custom ‘layout’: the layout of the menu components (game icons, user icons, time, etc.). That custom layout removes a few of the app icons from the home menu to make it more minimal. There is no way to access these. The Switch simply doesn’t display the all-apps button when you have less than 13 apps installed.