Unfortunately there is no way to resolve this from the modeler.
You need to download Tortoise SVN and then resolve the conflict manually https://tortoisesvn.net/ . Open the css file in a notepad and then go over the changes with your collegue.
After you are done with changes to the css file you need to signal to SVN that the conflict is now resolved.
When you right click on the conflicted file in explorer you should see the TortoiseSVN submenu. In the context menu you should see a mark as resolved button that you need to click.
Finally don't forget to commit after you have resolved the conflict, so that no other developer run in the same issue.
Hope this helps,