[material theme] Fix cm-error style

it seems `.cm-error` lost some specificity in [this commit](https://github.com/codemirror/CodeMirror/commit/a7d443ec82c1806d24e25ad8014a4d8651409320#diff-33646e9ee4d04eedb98a991a636c2302L35) that causes the error state to have same color as the tag.