The following CSS code "fixes" the colorful sidebar in Gitter, turning the background dark-gray-blue, similar to Discord.
.panel-footer__item--all {
background: linear-gradient(to bottom, rgba(0,0,0,0) 0%, #36393E 90%);
}
.room-menu__panel:before {
background: #36393E;
}
To use this you need an extension for your browser to apply custom CSS code to websites. On Chrome I'm using Stylebot, but there should be several extensions for all major browsers to allow something like this.