Light theme polish

[subsymbolic]

Task/Issue URL: https://app.asana.com/0/361428290920652/832439960662903

Description:
Light theme polish

Steps to test this PR:
NOTE: it may be helpful to comment out the sa, sb and mg variants in VariantManager.kt.

Light theme tweaks:

1. Go to settings
2. Switch to light theme
3. Note the gray background of the toggles show
4. Press the up button, note the ripple/shadow shows
5. Visit the tabs switcher screen, note the lighter background.
6. Note also that the unselected tabs are no longer grayed out (this also applies in dark mode)

Ensure omnibar does not disappear

1. Open the settings and switch between light and dark modes a few times
2. Press the back button to go back to the browser screen
3. Ensure the omnibar is still visible

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Internal references:

Software Engineering Expectations
Technical Design Template

[brindy]

Looks good - the status bar is dark when switching from light to dark, but that seems to only be problem on a small number of emulators/devices.

[brindy]

Hold off merging - the overflow menu on the bookmarks screen is black text on dark background. Tried it on an API 23 and API 27 emulator...

[subsymbolic]

Good catch, I have updated the overflow menu color. Thanks for the review @brindy.

[brindy]

LGTM!