The Chat widget uses it’s own default styling, defined within the _variables.scss of the module. If you want to make adjustments to your styling to fit your own custom needs, this can be done by adding an exclusion to your theme; for example.
$exclude-chat-variables: true;
Then duplicate the _variables.scss file, place it into an _extended module and make adjustments when neseccary. Using this method; when there is an update of the BizzomateChat module, your custom styling will not be overwritten.