C# - Not All Css Files From The Selected Theme Are Included Into The Page?

I have a Asp.Net Web Application that uses Themes. Inside the theme directory are 9 *.css files.

I've published the Web Application months ago on a IIS 7.5 (and updated regularry). If I browse the site from my computer, my private computer or some virtual machines at a hosting facility, the site will be perfectly rendered.

However, some employees of the company we created the page for are recently having display issues. I was just able to reproduce the issue via remote session and found out, that only 8 of the 9 styles in the selected Theme directory are included into the page. (IE developer toolbar showed the this)
The client is using Internet Explorer in version 8.

The problem does not only seem to affect employees of the same company rather than visitors from outside as well.The name of the stylesheet, that sometimes not included into the page is called style.css.
I've checked if the file was not accessible from the computer I created the remote session with, but that was not the case. The style.css was browsable. For now I'd try to move the style.css out of the App_Themes directory and include it manually via the MasterPage.

Update: I also discovered that some

<asp:Images ImageURl="~/images/mypic.png">

are also not being shown, just the red X for image not found. Maybe that's related? I looked for the Internet Explorer setting multimedia -> show images, it was checked. I'll try to replace the pngs with jpgs for now.