Hi I'm new to SP 2010, picking up a site someone else customized.

I opened the site's master page then followed the link to the css section to change.

Saved the change and the page looked fine. But, I went back to make another change and the original change was overwritten!

This is only 1 site, 1 master page, but i noticed something weird - there are multiple core.css files in the top-level site, all in weird randomly named folders in {top-level site}/_catalogs/theme/themed/1386CE2C/corev4-80A8FAC2.css

I'm only familiar with SP 2007 customization, so I'm not quite sure what's going on here.

Can someone try to explain or point me to a link that will help? thanks!

3 Answers 3


This was a good read:

Themable CSS Registration and Rendering for SharePoint 2010


In SharePoint 2010, the Core.CSS file is automatically generated with Themes.

The CSS base files are populated with lots of attributes that are parsed when a theme is set, thus creating a new Core.CSS file which has the elements for a specific theme dropped in.

What you're better doing, is creating a new CSS file from scratch, and filling in your customisatiosn there, then add it to the masterpage.

As a general rule, never modify anything SharePoint gives to you in the 12 hive (SharePoint Root in 2010). Changes will more often than not just be overwritten, especialyl with a service pack installation.


ahum the 14 hive ;-)

  • ahum, nope the core.css in 14 hive has not been modified. but i can see when i open core{randomletters}.css that there's additional CSS at the bottom.
    – user2330
    Commented Feb 17, 2011 at 22:33

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