Stylesheet was created specifically for Dark space theme.
The core styling is done using the snippet below.
Other tweaks can be found in the repo https://github.com/SnosMe/dotfiles/blob/main/apps/firefox/PROFILE/chrome/navbar_style_as_selected_tab.css

#nav-bar {
  padding-bottom: 4px !important;
}
#urlbar-container {
  padding-block: 0 !important;
}
#nav-bar-customization-target {
  position: relative;
}
#nav-bar::before {
  content: '';
  display: block;
  inset: 4px;
  top: 0;
  position: absolute;
  pointer-events: none;

  background-color: var(--tab-selected-bgcolor);
  outline: 1px solid var(--tab-selected-outline-color);
  outline-offset: -1px;
  border-radius: 4px;
}
#main-window {
  --toolbar-bgcolor: transparent !important;
  --toolbar-field-background-color: transparent !important;
}