Normalize background color

[EvanHahn]

Most browsers set white as the default background, but not all. When reading in "night mode", Pocket defaults to having a dark background for web pages. This renders some pages unreadable. This aims to fix that for sites using normalize.css.

This PR doesn't update version numbers or the changelog, but it can!

[battaglr]

If I'm not wrong, this was already done and resulted problematic, check #193.

[EvanHahn]

Why wouldn't normalize.css be the first thing you include in your CSS? (This is an ignorance question, not a foot-stomping "why don't you like my idea" question.)

[necolas]

More details in: 71d6cdc

Example of when you might have CSS before normalize: user/page-specific settings that are dynamically rendered in a <style> block before your main stylesheet. We had this requirement when making Tweet Cards for Twitter, where the base link styles and background color needed to be dynamically based on the styles in the host app (e.g., twitter.com, TweetDeck, Syndicated Tweets).

[EvanHahn]

Okay. Sounds good!