So there are lots of approaches to managing CSS, and there’s really no “best way” unfortunately, both in the broader SPA world and in Ember specifically. In my experience managing css is one of the things that is really hard well to do long term. I’m a little cynical about it though. We’ve had a lot of discussion around it recently at my company and mostly it’s just shrugs.
That said there are some great tools in the ember ecosystem so while I can’t point to one specifically I can recommend a few and you can look at them and see what makes the most sense for you. Hopefully others can weigh as well.
There are a couple addons that deal with what I would call “auto scoping” of CSS. ember-component-css is great for components (and only components). Essentially what it does is automagically create unique classes for each component and apply the correct stylesheet, guaranteeing that the styles are scoped only to the component. ember-css-modules allows you to break up your styles to correspond with your js and hbs files, and it also provides the “local-class” mechanism for scoping your css. These save you the trouble of setting up all the sass imports and class prefixes manually (what we do at my company currently) and that helps keep the styles in sync with the other code. I’m not sure what the status of either of these is in terms of module unification though, especially since MU is changing/delayed.
It appears that you’re already using (or planning to use) sass but I’ll go ahead and note that in addition to ember-cli-sass and ember-cli-less and similar libs there’s also an addon for PostCSS which can be really nice for not only preprocessing but adding other plugins like autoprefixer, etc.
One thing which I’ll also pitch to you as a potential option is functional css. If you’re not familiar with it it’s basically “utility class first” CSS meaning instead of having separate stylesheets you just attach a bunch of utility classes to your HTML markup. It’s controversial but a lot of people (myself included) really love it. Tailwind is probably the most popular functional css lib and ember-cli-tailwind is a fantastic ember integration. This is purely anecdotal but IMHO it makes CSS a lot more fun and maintainable long term (removes the need to name things and keep separate stylesheets and so on). I just made a little demo app and slide deck recently to present to our engineering teams which you can take a look at if you’re interested (the slides are in slides.html, may need to clone the repo to see them correctly).
Anyway if you have any questions on any or all of the above let me know, I’d be happy to dive deeper into any of it.