Custom redactor.css file
You can override the control panel's CSS with a plugin. Use the popular "Control Panel CSS plugin" by Lindsey DiLoreto or make your own little one.

Make use of the includeCssResource() function and include a CSS file with your custom redactor styles:

myRedactorCssPlugin.php:

public function init()
{
    if (craft()-&gt;request-&gt;isCpRequest())
    {
        craft()-&gt;templates-&gt;includeCssResource('myRedactorCss/redactor.css');
    }
}


resources/redactor.css:

.redactor_editor h1 {
  font-weight: normal !important;
}
.redactor_editor p {
  background-color: #eee;
}