brocken compiling with less v4

to support LESS v4 you must wrap math operations with `(...)`

example 
```
    margin-top: (-@handleSize / 2);
    padding-top: @handleSize / 2;
```

give back 
```
    margin-top: -16px;
    padding-top: 32px/2;
```

and in this case `padding-top` will be ignored in browser and `margin-top` is correct!

please fix this asap!