Better theme handling

Release notes for 3.0.0

So many nice new things!

• More stable
• Faster
• Less memory hungry
• Styling of picture-elements
• Theme styling of sidebar
• Changing icons and icon color
• GUI support

To access the new cool features, you need to replace

style:
  ... stuff here ...

with

card_mod:
  style:
    ... stuff here ...

Just style should still mostly work, but you won't get the new features.

Detailed behavior may be slightly different in some edge cases.

Read the readme.