Fix BASE16_THEME env set by fish shell helper

[psaints]

The fish shell helper script was using the wrong variable to parse the current theme name from the theme's path, thus setting the BASE16_THEME environment variable to an empty string.

This affects scripts or configurations that rely on BASE16_THEME, such as the base16-vim users that use the instructions on the README file (colorschema base16-$BASE16_THEME).

[jcelliott]

I had the same issue. This change works for me.