Fix CSS variable name + test to prevent failures.

[isoos]

Fixes #8106

[jonasfj]

No strong feelings, just a slight concern about the churn we add here.

But I can also see more churn by adding more linters/tools that everyone will have to configure/install.

[isoos]

Having an unused variable is pretty harmless is it not?

I'd consider it the same as unused code/variable lints in the Dart code. Since we filter out those, we should filter out the same here.