Add light theme pixels

[subsymbolic]

Task/Issue URL: https://app.asana.com/0/361428290920652/816426578517930
Tech Design URL: https://app.asana.com/0/361428290920652/814618797160182

Description:
Adds pixel for settings and switching between light and dark theme

Steps to test this PR:

1. Comment out all variants other than mh and mj in VariantManager
2. Install the app
3. Open Settings note that the mp pixel fires
4. Toggle the theme a few times and note that the mp_tl (light theme) and mp td (dark theme) fire as appropriate

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Internal references:

Software Engineering Expectations
Technical Design Template

[brindy]

Looks good - something to consider though is that there are other things on the settings screen. You might see a lot of "open settings" pixels and if you don't see many of the toggle light/dark theme ones it might be the user is going to settings for something else. Ideally you'd add a pixel to everything so that you can apportion the opening of the settings with toggling (and everything else), otherwise it's going to be pretty meaningless when looking at the data.

[subsymbolic]

At this stage we want to know if users ever open settings so that we can work out if the toggle is in a discoverable enough place, I'm sure we'll add more pixels later. In terms of light / dark toggle the value selected will be compared to each other to see if users are more likely to switch to light or dark (half users were set to light and the other to dark by default)