Add a query parameter to embed URL to control light vs dark theme

From tpo/web/snowflake#4

Can we implement this?

• We can add query to the web address to control the badge's look:
  • If we go to snowflake.torproject.org/embed.html?_theme=dark_ the badge will be dark.
  • And if we use snowflake.torproject.org/embed.html?_theme=light_ the badge will be light.
  • When we don't add anything, the badge will still be dark by default.
• Doing so, People can choose if they want the badge to be light or dark.