There have been discussions that @import should not be used. For one, it adds a layer to the upkeep of style sheets. More importantly, it adds delays in the loading of style sheets.
A bit of history of @import.
The "@import" rule was introduced due to Netscape Navigator 4.x
As Netscape at the time didn't support all of the CSS syntax that was available there were always problems with layouts not working correctly in their browser. A workaround was introduced using the "@import" rule. The idea behind this rule is to create two separate CSS files, one which included CSS syntax that was understandable by all browsers and one that had the CSS syntax in it that wasn't understood by the Netscape Navigator.
As the "@import" rule was not readable for NN4.x it overlooked the code and read only the standard rules that were provided.
Since adding the base style sheet to your custom template overrides the base style sheet, just include the base style sheet in your template and add the bits to the end of that style sheet that Nathan indicated.
Or, as you indicated, you can add it to the base style sheet. However, please check your templates to see if they include the style sheet in the template as if that is the case, it will not pick up the additions from the base template.