Fix deprecation warnings of Stylelint

[patric-eberle]

When running npm run test, there are several deprecation warnings shown by Stylelint. We should check how to resolve this issues, so that no dependency related issues are shown. Maybe we also need a code formatter to get around this issue, but still get the same result.

Deprecation warnings:
 - The "block-closing-brace-newline-after" rule is deprecated.
 - The "max-line-length" rule is deprecated.
 - The "no-eol-whitespace" rule is deprecated.
 - The "selector-list-comma-newline-after" rule is deprecated.
 - The "string-quotes" rule is deprecated.
 - The "indentation" rule is deprecated.