Shade color interpolation
I think users should only need to give the darkest color and the lightest color for the shades. Themer should automatically generate the in-between colors using this formula:

```
DARK = someRGB
LIGHT = someRGB
N = number of colors we want (8 for themer)
DELTA = (LIGHT-DARK)/(N-1)
COLORS = [DARK+(DELTA*i) for i in range(N)]
```