UserChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* ------------------------------------------------- */

/* Adjust Colors */
.tab-background:not([selected=true]),
.tab-background[selected="true"],
/* #titlebar-buttonbox, */
#TabsToolbar,
/* #PersonalToolbar, */
#urlbar,
#nav-bar {
  background: #31363b !important;
}


/* ------------------------------------------------- */

/* Removes min/max/close buttons */


/* ------------------------------------------------- */

/* Rounded url bar */
#urlbar, .searchbar-textbox{
    border-radius: 2em !important;
}

/* ------------------------------------------------- */

/* Fix new tab flash */
.browserContainer { background-color: #202124 !important; }

/* ------------------------------------------------- */

/* Hide Back and Forward buttons */
#back-button, #forward-button {
  display: none !important;
}

/* ------------------------------------------------- */

/* Hides the url focus color */
/* Hide urlbar boarder */
#urlbar {
  border: none !important;
  border-color: transparent !important;
  border-width: 0px !important;
}

/* Clean Up */
.urlbar-icon,
.tab-close-button.tab-close-button,
#page-action-buttons,
#identity-box {
  display: none;
}
#urlbar .urlbar-input-box {
    text-align: center;
}
.urlbar-input-box > .urlbar-input::placeholder {
  opacity: 0 !important;
}

/* Hides the url go button */
.urlbar-go-button {
  display: none !important;
}

/* ------------------------------------------------- */

/* Remove border under navbar */
#navigator-toolbox::after {
  border-bottom: 0px !important;
  }

/* ------------------------------------------------- */

/* Remove Tab Borders */
.tabbrowser-tab::after, .tabbrowser-tab::before {
   border-left: none !important;
}
:root {
  --tabs-border: transparent !important;
}

/* Hide blue stripe on active tab */
.tab-line[selected="true"] {
  opacity: 0 !important;
}