That’s actually also how Yii works, if you didn’t know it:
The default UI framework for Yii is Bootstrap.
That means that people can use CSS, or they can use Less (for current Bootstrap) or the Sass official Bootstrap fork.
(Bootstrap 4 is Sass).
There are lots of Yii boilerplate templates available - you can choose the one provided by Yee CMS, or one of the others.
Should Yii have a collection of official templates/extensions ?
As for the ‘basic’ application template, do you want this to be offered in Sass? -> https://github.com/yiisoft/yii2-app-basic/blob/master/web/css/site.css
DISCLAIMER:
My word is not Gospel - even though I am talkative.
But I have been around long enough to know what is Yii.
And it sure is not Laravel 
No, what Yii needs is a better overview of what’s out there.
Not something that is pre-packaged.
If you want a turn-key solution, why not use Yee CMS ?
Or some of the other alternatives?
And, that’s my point: it is already in place.
The Yii community is large, and there is something for everyone.
There’s even a full fledged, professional grade CMS that you can just install and use: Craft CMS
The community is already providing, so that the Yii team can concentrate on Yii itself.
And that is all that I want to say about this topic - I think I have said more than enough already 
Let other people chip in and speak their mind.