Notebook 7 and custom.css

Notebook
6

Notebook 7 uses CodeMirror v6. .CodeMirror-matchingbracket is not the correct class name for CodeMirror 6. It should be .cm-matchingBracket. Then, your code includes :hover which means it would be only applied on hover. .CodeMirror is also an old class name, in CodeMirror 6 it is .cm-editor. The following works for me:

.jp-CodeCell .cm-editor .cm-matchingBracket {
   color: red!important;
}
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