Allow user to keep the homepage when selecting a new theme

Description

When someone decides to change their theme, they may have already customized the homepage. Instead of automatically overwriting it, we need to give them an option to keep their existing one.

To achieve this we will add one more step after the user chooses to activate a new theme. This will prompt the user to select between two options:

• Use the [New Theme]'s homepage layout
• Keep current homepage layout (default preselected option)

The user may press the Preview button to view the new theme in action.
Hitting the Activate button will apply the new theme respecting the user's choice.

Note that this functionality already exists on the web.

Design

The new step will be presented as a bottom sheet sliding from the bottom. The design below depicts the theme change flow with the addition of the new step.

Note that the design above is targeting iOS but the same changes apply on Android too. Our design team can provide clarifications or extra designs if needed.

There is a matching iOS issue here.

Trace the Android theme activation flow and compare it with the linked iOS issue and the provided design. Add the bottom-sheet choice between the new theme homepage and the current homepage, with the current layout selected by default, then verify Preview and Activate respect that choice.