feat: introduce theme config and add qunit-theme-ember as option

[IgnaceMaes]

Introduces a theme config option and adds qunit-theme-ember as option.

Before	After

In a follow-up PR, this can be made the default. This can first be released as a minor, and switching the default to Ember would be a major.

[acorncom]

@IgnaceMaes great idea, I like it a lot! ❤️ My guess would be this would be considered a breaking change (I'm working with a client right now that would break if this ended up in their app) as they have custom CSS and assumptions about the qunit theme that wouldn't be valid any more. But the idea of a facelift to help Ember look modern is great.

Think you could write up a brief RFC? I could see the default needing to be qunit until Ember 6, with a deprecation notice if you haven't specifically chosen an option. When we switch to Ember 6 we could flip the default to your new theme (and potentially have done the switch in blueprints earlier than that). Or something along those lines ...

[patricklx]

Well, it can then default to old qunit-style. Having the option to easily switch to new style?

[ijlee2]

Having the option to easily switch to new style?

I'd also suggest an opt-in. Another library that may be affected if we change the default style is @percy/ember. At the moment, it could be hard to introduce changes to that repo.

[IgnaceMaes]

Thanks for the input, everyone!

The way to go forward seems indeed making the theme configurable. If the default is to be changed, it should go through an RFC and only be released in a next major version.

[ef4]

Add this behind an option does not require an RFC. Flipping the option on by default would arguably deserve an RFC.

(I came here because of emberjs/rfcs#1017, which seems possibly unnecessary, at least to make this an option.)

[IgnaceMaes]

Thanks for clarifying.

Next steps:

• Split up this PR to make the theme configurable (independent of qunit-theme-ember)
• PR to make qunit-theme-ember the default

[wagenet]

I'm not doubting, but I'm curious what people are doing that would be broken by a theme change.