I'm note even sure what @import url(../../base/css/icon.css); ??? is.
What the folks are saying is, any file in your custom template will override a file with the same name in the Boonex template base folder.
So, grab the icons.css file from the base/css/ folder and place it in your templates/custom_template_name/css folder. Make your changes there and they'll override the code in base.css when the script is run.
As you've already made changes, you can just copy the file. However, in future only update the icon.css file in your custom template folder.
Future updates of Dolphin MAY replace the base/icons.css file and all your changes will be lost if you make them there. Dolphin updates WON'T replace your custom template folder.
I hope that makes sense. In theory, when Dolphin runs, it will read ALL the code in Template/Base folder, then the code in your custom template folder making on-the-fly changes as necessary.