In most cases custom CSS is located inside the “Customizer” portion of the theme. Abacado deviates from this and has placed this inside the Acabado Settings Menu.
This can lead to you not knowing where to go to place your content to ensure your tweaks get added. Please remember the location and as you add and customize this section take a backup of the customization you include in the case you should ever need to restore from a backup.
This custom CSS section should carry over from upgrade to upgrade, please for your own sake make a text backup of this so you can re-apply or you may have to spend hours finding and changing them all over again.