Hi Mark,
The problem is that images are relative to the location of the css file. The the themeStyles.css file is NORMALLY located at
/modules/xerte/parent_templates/Nottingham/common_html5/css/themeStyles.css
and therefore the reference to the, for example, maximise icon in the themeStyles.css file is ../maximise.png which equates to
/modules/xerte/parent_templates/Nottingham/common_html5/maximise.png
If however you upload a new css file with ../maximise.png in it then this will refer to the actual data folder of that particular LO. Therefore you have two options, either:
1) replace the ../ reference with the fully qualified URL - http://DOMAIN/modules/xerte/parent_templates/Nottingham/common_html5/maximise.png OR
2) replace the ../maximise.png with just maximise.png and then re-upload all the icons as media elements
The reason that you need the DOMAIN part is that if you go to export the LO then media referenced outwith the data folder WILL NOT be included so you need a reference back to the original website. Alternatively, if you re-upload the icons then these WILL BE included in an export. You also have the added bonus that you can also customise the icons on a per LO basis...
Hope that helps,
John