You should be using the EL variable #{resource} in CSS to specify image resources.

Change below lines for bootstrap.css for example

@font-face {
  font-family: 'Glyphicons Halflings';
  src: url('../fonts/glyphicons-halflings-regular.eot');
  src: url('../fonts/glyphicons-halflings-regular.eot?#iefix') format('embedded-opentype'), url('../fonts/glyphicons-halflings-regular.woff') format('woff'), url('../fonts/glyphicons-halflings-regular.ttf') format('truetype'), url('../fonts/glyphicons-halflings-regular.svg#glyphicons-halflingsregular') format('svg');
}

to

@font-face {
  font-family: 'Glyphicons Halflings';
  src: url('#{resource["default:bootstrap/fonts/glyphicons-halflings-regular.eot"]}');
  src: url('#{resource["default:bootstrap/fonts/glyphicons-halflings-regular.eot?#iefix"]}') format('embedded-opentype'), url('#{resource["default:bootstrap/fonts/glyphicons-halflings-regular.woff"]}') format('woff'), url('#{resource["default:bootstrap/fonts/glyphicons-halflings-regular.ttf"]}') format('truetype'), url('#{resource["default:bootstrap/fonts/glyphicons-halflings-regular.svg#glyphicons-halflingsregular"]}') format('svg');
}