Another font and another background

[simonhamp]

This was going to be a simple PR to add an image and a font, but I realised a few things whilst testing these additions out, so now this is a ⚠️ breaking change, but I think it's for the best at this early stage... hopefully this will be the last breaking change before v1!

Added

• GridMe background (specifically for @stillat. Tweet)
• RobotoSlab font

Here they are in action

Sneaker photo by Matthew Hamilton on Unsplash

Changed

• Removed AbstractTheme in favour of simply instantiating the Theme class.

If you were extending AbstractTheme, now you can simply create an instance of SimonHamp\TheOg\Theme\Theme instead.