SplitButton's isOn=true state is barely visible.

[jodator]

The same goes for actionView:

and arrowView:

[jodator]

Adding:

& > .ck-button,
& > * > .ck-button,
& > * > * > .ck-button {

in https://github.com/ckeditor/ckeditor5-theme-lark/blob/7999134bef289f75e827d83b531e5b1aeaa71f22/theme/ckeditor5-ui/components/editorui/editorui.css#L39-L49

Fixes this but might be not the best solution:

[oleq]

The problem has been analyzed correctly. That's more or less what we're supposed to do.

But still that's a tricky one, I'm afraid. I've never liked this Christmas Tree selectors but I couldn't come up with a better one. Ideas:

Decentralization

We could move this logic to components/features – a drop-down would add such style and we could even provide a mixin for this (delivered by editorui). But then, these styles are not very useful, if someone decided to compile the theme without the EditorUiView (e.g. replaced with something else?). It's an upside-down approach anyway, a drop-down (child) understanding stuff about a hypothetical parent (toolbar). A no-go for me.

Narrowing down

Instead of & > * > * ... we could simply put all use cases here like

& > .ck-button,
& > .ck-dropdown > .ck-button,
& > .something-to-do-with-split-button > .ck-button {

There are not many of them. Would there be more (like much more)? I don't think so. It would also make more sense for developers decyphering this code because it clearly states what cases it covers.

LGTM

Another class

A perfect solution would be a CSS class (e.g. .ck-toolbar__button) for such buttons to be styled. But honestly, I see absolutely no way to add it to a dropdown, which is hosted by the toolbar. A toolbar would need to know that its child's button (div.ck-dropdown > .ck-button) should get such class. How? No idea. But it would definitely be the most elegant approach.

cc @Reinmar @dkonopka

[jodator]

So it should be like this:

& > .ck-button,
& > .ck-dropdown > .ck-button,
& > .ck-dropdown > .ck-splitbutton > .ck-button {

I'm fine with that as it is more clear what it does then Xmas tree before.