Adds typography, breakpoints, icons, adjusts dark mode colors

This release includes styles for typography and more.

First up is typography. We standardized our typography and put the styles in this package. Our headers are now based on a major-third scale. Our body text now stays the same size across large, medium, and small screens, so that text never gets too small to read.

We also added breakpoints: small, medium, and large.

Our icon set is now larger than before and includes icons from some Expo-specific things, like our plan icons.

Finally, we added support for switching between palettes automatically with our theme switching CSS variables. We now expose theme colors like theme.palette.blue[200]. When in light mode, this will display theme.light.blue[200] and when in dark mode it will present theme.dark.blue[200]. This is possible because lowered the saturation on our dark mode colors. These are great for illustrations and decorative elements.

We also added theme.background.screen, which will show as a light gray color in light mode, and nearly black in dark mode. It's great when used as the background "screen" color in a layout.