userChrome.css

/* Fixing back button in normal density */
:root:not([uidensity=compact]) #back-button {
    padding-top: 0 !important;
    padding-bottom: 0 !important;
}

:root:not([uidensity=compact]) #back-button:not([disabled]):not([open]):hover > .toolbarbutton-icon {
	background-color: var(--toolbarbutton-hover-background) !important;
	border-radius: var(--toolbarbutton-border-radius) !important;
}

:root:not([uidensity=compact]) #back-button > .toolbarbutton-icon {
    background-color: unset !important;
    border: none !important;
    border-radius: 0 !important;
	box-shadow: none !important;
	width: calc(2 * var(--toolbarbutton-inner-padding) + 16px) !important;
	height: calc(2 * var(--toolbarbutton-inner-padding) + 16px) !important;
}

/* Remove extra space around tabs */
.titlebar-spacer[type="pre-tabs"] {
    width: 25px !important;
}
.titlebar-spacer[type="post-tabs"] {
    width: 5px !important;
}

/* Remove tab list icon */
#alltabs-button {
  display: none !important;
}


/* OPTIONAL */
/* Remove unnecessary identity-box content */
#urlbar[pageproxystate="valid"] > #identity-box.verifiedDomain > #connection-icon,
#urlbar[pageproxystate="valid"] > #identity-box.verifiedIdentity > #connection-icon,
#urlbar[pageproxystate="valid"] > #identity-box.mixedActiveBlocked > #connection-icon,
#urlbar[pageproxystate="valid"] > #identity-box.verifiedIdentity > #identity-icon-labels {
  display: none !important;
}
#urlbar[pageproxystate=valid] > #identity-box.notSecureText,
#urlbar[pageproxystate=valid] > #identity-box.verifiedIdentity,
#urlbar[pageproxystate=valid] > #identity-box.chromeUI,
#urlbar[pageproxystate=valid] > #identity-box.extensionPage,
#urlbar-display-box {
  border: none !important;
  margin-right: 0 !important;
}

changed look of #back-button to match #forward-button