I didn’t got your question fully, anyhow Go and check your Htmltemplate folder in CSS file you need to edit, and in index.html or main.php you need to change
I have the same question. I can add the tag manually in the layout, as Lukos suggest, but there’re not reference to bootstrap or other css styles there, so Yii is loading them in another way. Wich is that way? Can be used to load custom css too? I think this is what Diego asked!
EDIT: May be it has something to do with AssetBundles.