Good catch. Something needs to be done when adding theme + ggplot.

I experimented with removing __add__. Without it, it is not possible to add two themes together using '+'.  __radd__ only comes into play when there are different types. Without __add__
```
    t3 = t1 + t2
TypeError: unsupported operand type(s) for +: 'theme' and 'theme'
```
I can raise a TypeError in theme.__add__ if other is ggplot. Would that work? It looks like this. Also added a unit test for this.
```
    theme() + gg
TypeError: unsupported operand type(s) for +: 'theme' and 'ggplot'
```