Responsive topics (pages)

It is possible to add some additional CSS code into topic pages?
For example, I'm implementing an Iframe into one of my pages using the "source code viewer". This works fine, but I'd need to add some CSS to make it more responsive on mobile devices.

I could add some div's to the source code of the topic and style it with the general theme, but maybe there are better solutions available that doesn't require a new theme version?

go Configurations -> Settings -> All settings (advanced)
to enable style
adminareasettings.richeditorallowstyletag = true
to enable  js
adminareasettings.richeditorallowjavascript = true
edit the topic source code  and put those from view/tools menu of rich editor

<script type="text/javascript">alert('test');</script>
<style type="text/css">p{color:red}</style>