Highlighting breaks in single-quoted DEFAULT_CSS

See `test_dom.py::test__get_default_css`:

```py
def test__get_default_css():
    class A(DOMNode):
        pass

    class B(A):
        pass

    class C(B):
        DEFAULT_CSS = "C"

    class D(C):
        pass

    class E(D):
        DEFAULT_CSS = "E"

# ...
```