Remove circle around back button (Nightly 57)

[Skrity]

I dunno how to make PR so I figured I'll contribute this way.
Tested on W10/57.0a1 (2017-09-03) (64-bit). Not sure if complete, also probably hacky, so if someone can make it better you are welcome 👍

/* Remove circle, background, padding from backbutton */
:root:not([uidensity="compact"]) #back-button > .toolbarbutton-icon {
    background-color: unset !important;
    border: unset !important;
    padding: var(--toolbarbutton-inner-padding) !important;
    border-radius: var(--toolbarbutton-border-radius) !important; }
/* Hover and Active states to mimic other buttons */
:root:not([uidensity="compact"]) #back-button:hover:not([disabled]) > .toolbarbutton-icon {
    background-color: var(--toolbarbutton-hover-background) !important; }
:root:not([uidensity="compact"]) #back-button:active:not([disabled]) > .toolbarbutton-icon {
    background-color: var(--toolbarbutton-active-background) !important; }
/* Remove pre-padding */
:root:not([uidensity="compact"]) #back-button {
    /*padding-top: unset !important;
    padding-bottom: unset !important;   /* These two will screw nav-bar height (Nightly57)*/
    padding-inline-start: 3px !important; }

[Timvde]

Thank you for contributing! Is this style meant for Firefox 57+ only? It also seems to work for Australis, but it's not as pretty.

[Skrity]

Yep I use nightly as a daily driver, so I only tested there. But it works on all out-of-box lightweight themes.

[Timvde]

Added in 2037fbb :) Thanks again!