Alternatively use SASS (and the bootstrap SASS port[1]) which doesn't has this problem, as it instead rewrites selectors.

i.e. if you have:

    .column {
      width: 100px;
      display: inline-block;
    }

And you do:

    .cool-sidebar {
      @extend .column;
    }

Then the generated css will be something like:

    .column, .cool-sidebar {
      width: 100px;
      display: inline-block;
    }

[1] https://github.com/twbs/bootstrap-sass