Add theme hook

[shangzhel]

• Closes Add theme hook #118

Adds:

• Palette
• Shapes
• Font families
• Font weights
• Font sizes
• Icons
• Code samples (does anyone actually want this?)

[chomosuke]

I'm guessing useTheme will return default theme unless overwritten by some ThemeContext higher up in the hierachy. Therefore we should always use useTheme to get our theme.

Unless we explicitly want the default theme.

[oldbugo]

Nice, thanks for the implementation. One thing we might want to consider is changing the naming scheme for the font-size from 16, 24, 36, to small, standard, title, just so that if we were to change the size itself the naming convention won't need changing. But I also think this naming may be confusing to use so I'll leave it up to you if want to make this small change. Approved either way.

[oldbugo]

I've noticed I have missed some icons in the style guide, so after discussing with Richard we made some changes together to include the new icons and deleted some no-longer-used icons. And we made some changes to naming scheme of the variables to be more flexible if we were to change it in the future. Please have a look at richard/theme-hook

[oldbugo]

Thanks