This is probably due to the CSS caching.
If that's the case, to fix this issue you'll need to add a query parameter to the CSS file call, for instance add "?v=1" on line 115 of the icon-blocks.sfMarkup:
<Style name="imageBlockBaseCSS" href="resources/image-blocks-base.css?v=1"></Style>
This will cause the browser to pull in the latest changes.
I hope this helps,