Can't customize vim text color under cursor

I’m trying to change the color under the cursor in vim (visual mode), with no avail. I’ve used the same syntax found in this thread, but it looks like it doesn’t work.

This is my theme.css:

.cm-fat-cursor .CodeMirror-cursor {
    background-color: #ff0000 !important;
    color: #000000 !important;
    opacity: 60% !important;
}

.cm-animate-fat-cursor .CodeMirror-cursor {
    background-color: #ff0000 !important;
    color: #000000 !important;
    opacity: 60% !important;
}

While this is the result:

Screen Shot 2021-01-09 at 12.34.41

The expected result would the text color, under the cursor, to be black instead. Like this:

Screen Shot 2021-01-09 at 12.42.20