Often when you make a theme, styles will affect the editing interface for c5. This is because the styles are 'greedy' - say for an h1 it applies to all h1s when you are editing. If you add a wrapper around all of your content, then this will stop happening. Make sure to use a div or other block level element rather than a span to prevent semantic errors in IE. To make that even easier, use a css preprocessor and wrap all of your includes and custom rules inside of it!
<body> <div class="scope-wrap"> <!-- All of your content here --> </div> </body>