UserChrome

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#TabsToolbar {
    visibility: collapse !important;
}

/*Collapse in default state and add transition*/
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] {
  overflow: hidden;
  min-width: 40px;
  max-width: 40px;
  transition: all 0.2s ease;
  border-right: 1px solid #0c0c0d;
  z-index: 2;
}

/*Expand to 260px on hover*/
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"]:hover,
#sidebar-box[sidebarcommand="treestyletab_piro_sakura_ne_jp-sidebar-action"] #sidebar {
  min-width: 260px !important;
  max-width: 260px !important;
}

UserContent.css

/* Removes white loading page */

@-moz-document url(about:blank), url(about:newtab), url(about:home) {
    html:not(#ublock0-epicker), html:not(#ublock0-epicker) body, #newtab-customize-overlay {
      background: #1d2021 !important;
    }
  }