publicPath in webpack.config.js seems to be wrong

[hubionmac]

I but my fonts into the asset-folder
e.g:
/themes/mythemename/assets/fonts/open-sans-v18-latin-300.woff

in /themes/mythemename/assets/css/theme.pcsss
I put

font-face {
  font-family: 'Open Sans';
  font-style: normal;
  font-weight: 300;
  src: url('../fonts/open-sans-v18-latin-300.eot'); /* IE9 Compat Modes */
  src: local(''),
       url('../fonts/open-sans-v18-latin-300.eot?#iefix') format('embedded-opentype'), /* IE6-IE8 */
       url('../fonts/open-sans-v18-latin-300.woff2') format('woff2'), /* Super Modern Browsers */
       url('../fonts/open-sans-v18-latin-300.woff') format('woff'), /* Modern Browsers */
       url('../fonts/open-sans-v18-latin-300.ttf') format('truetype'), /* Safari, Android, iOS */
       url('../fonts/open-sans-v18-latin-300.svg#OpenSans') format('svg'); /* Legacy iOS */
}

but this results in a wrong path in the output css

  font-weight: 300;

  src: url(themes/mythemenameassets/fonts/open-sans-v18-latin-300.eot); /* IE9 Compat Modes */
.
.
.

but when I change publicPath inside the webpack.config.js to this
publicPath = '/'+ path.join('themes', data.themeName)+'/'
the path turns out fine and so far everything else as well ;-)