Set dark_mode_ui_toggle to on by default

[obulat]

Fixes

Related to #4305 by @zackkrida

Description

This PR sets the dark mode feature flag to on by default when it's switchable (in staging/local). This will enable running tests for light/dark color theme using the color mode toggle in the footer.

Testing Instructions

Run the app. You should see the color mode toggle in the footer by default. The snapshot updates should add the toggle in the footer.

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[zackkrida]

Looks good.