Remove light theme experiment and enable for all

[subsymbolic]

Task/Issue URL: https://app.asana.com/0/inbox/361428290920650/816830036286389/889504268656433
Tech Design URL: N/A

Description:
Remove the light theme A/B experiment and release the light theme as a settings option, maintaining the dark theme default.

Steps to test this PR:
Upgrade installation

1. Install the develop versions of the app, ensuring that you are allocated to varianr mj
2. Go to settings and switch to light theme
3. Now install this branch over the top
4. Ensure your light theme selection was maintained

New installation

1. Install a fresh version of the app
2. Ensure that the dark theme is the default
3. Ensure the theme can be changed to light in settings

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Internal references:

Software Engineering Expectations
Technical Design Template