can't multiply sequence by non-int of type 'float' when using label_font_size in custom style

Applying this custom style:

```
custom_style = Style(
background='#fff',
label_font_size='16',
value_label_font_size='16')
```
causes an error:

`can't multiply sequence by non-int of type 'float'`

This seems to be caused by the `label_font_size` option, since removing it makes everything work again.

Start by reproducing the custom Style example with string values for label_font_size and value_label_font_size, then trace how label_font_size is processed. Done means the example no longer raises the reported TypeError and a regression test covers these options.

Written by the indexing model from the issue text.