Fix Styles for Angular Material v15

[GeorgDangl]

I've just recently updated the dependencies to Angular v15. Material has lots of changes, so we need to remove the legacy elements and use the regular ones.

So we need to check that the components all still look good, and everything is working.

I've found one problem, with the cyan 600 theme used, the text color of buttons is black (but it should be white). It looks like there's a bug in Angular Material:
angular/components#26056 (comment)

I think this might be an idea to fix it:
angular/components#6244 (comment)

[GeorgDangl]

Now I did npm ci and it looked better😀

But I still seem to have an issue, I've got two buttons here (just added them):

Both have color="primary" set, but it only works on the top level one in the header, not on the one I've put in the app.components.html file in the demo project.

[GeorgDangl]

Here's an example of how the legacy button is used