You set your default theme in rwd/default. Magento has a call back functionality built in for theme management. That means it will look for a file in package/theme directory that you have set through admin. (If you didn't set any custom theme, then the default package theme would be rwd/default for Magento 1.9 and default/default for <magento 1.9 ) If the file isn't found there, it will check for that file in base/default folder.
So in this case, this call back functionality would somewhat work like this.
Browser requesting the url of css file to Magento.
Magento accepts request. It looks which is the current theme being used.
Magento recognizes current theme is rwd/theme. Bingo
Magento genrates the url based upon theme. It would be skin/frontend/rwd/default/css/main.css
Magento checks for that file there. Ooops !!! It is not there.
Magento thinks then let us check this file in base/default. So it generates a url skin/frontend/base/default/css/main.css and looks file there.
Urekaaa... File does exist. So magento includes this url for that css file
If the file is not in there, Magento become "desperate" and will say to us "Sorry, I couldn't find the requested file anywhere."(literally). So shows a no-found error.
So what do you need to do is, copy your main.css file from skin/frontend/base/default/css/main.css to skin/frontend/rwd/default/css/main.css. That will do the trick
http://127.0.0.1/mrcare-magentoon your browser address bar ?link href?rwd/default/csson the top two tags, however, on the one I included, it isbase/default/css, while it should actually be rwd, just like the other two.main.cssfile in the locationskin/frontend/rwd/default/css/main.css? You probably do not have that file there. But it exists inskin/frontend/base/default/css/main.css. Am I right ?