Remove Ruby specific syntax from filter attribute

Using `#{ruby_variable_interpolation}` means that this file can't be compiled using [node-sass](https://www.npmjs.org/package/node-sass).

Change the value to use Sass arithmetic, string concatenation and [the unquote method](http://sass-lang.com/documentation/Sass/Script/Functions.html#unquote-instance_method) to avoid this Ruby specific syntax.