Customize styles through UI API

[hmuurine]

Summary

Added code that allows the users to customize styles via the UI API.

• No longer applying styles automatically in constructor. Need to call applyStyles()
• Developers can give custom overriding CSS styles as optional parameters in new PixelStreamingApplicationStyle() constructor
• Moved light/dark mode palette applying code from Application to PixelStreamingApplicationStyle to keep all the style related code in a single place and separate from Application
• Application now exposes an optional onUpdateColors callback that is called when light/dark mode is toggled. The style class hooks into this to apply styles on mode change
• Developers can give light/dark mode palette as an optional parameter in new PixelStreamingApplicationStyle() constructor to override the default colors

Test plan

• Ran the frontend without any style overrides, and verified that the default styles were applied
  
• Toggled between light/dark mode and verified that the styles were applied
  
• Modified player.ts to apply custom styles and verified that the styles were applied
  
  
• Modified player.ts to set a custom light/dark mode palette and verified that the styles were applied on toggling color mode
  
  
  

[lukehb]

CC @Belchy06 for vis

[lukehb]

lgtm!