How to modify the CSS of the Google Hangouts extension window in Chrome (e.g. TamperMonkey)
Try a different extension. On Chrome, using the Stylish extension did not work for me. However, Osprey amd Stylus (with the exact same code) did. (The latter two are in fact just forks of Stylish &mdash; created because of privacy concerns related to Stylish now being part of an analytics company &mdash; but somehow they work whereas the original does not.)

The code I used for the style is the same as yours:

.Dkouue {
    display: none;
}