CSS Styling

- Refactored styling to be done via CSS variables rather than JavaScript during initialization
- Added CSS variable knobs to the demo page

Fixed buggy auto-sizing labels

Rewrote the hacky programmatic sizing. Replaced with simply programmatically setting known stepper width and step count, and let CSS use `calc()` to figure out stepper control widths and step title widths

Resize bugfix

- The resizing/recalculation of step labels now happens on initialization, and when the device's orientation is changed.
- The resize has now been smoothed to the default animation speed (can be changed
- Fixed bug causing extra padding on either side after a resize

Updated readme