[Screenshots] Inconsistency between light and dark mode on screenshots

[AliSoftware]

Parent: #17128

The set of screenshots generated by WPScreenshotTest are in a mix of light and dark mode.

In particular, in the current setup from the test code, most screenshots use dark mode, except the Notifications screenshot which uses light mode:

For consistency, we should probably use the same mode (either light or dark) for all screenshots in the Play Store, instead of this mix of both.

Note: Unlike WPScreenshotTest, the JPScreenshotTest seems to use light mode for all screens. So this tends to suggest we'd use light mode for all screenshots on WP too. Might be worth double-checking this and that we are consistent not only across all screens of a given app, but also across WP vs JP.

Note: Also, the screenshots currently on the Play Store (which were created manually by @shaunandrews the last time we updated them) seem to use light mode for all screens, so this furthers strengthen the suggestion for us to go in the direction to use light mode in the UI test code for all the generated screenshots.