Had a little play yesterday.
Dropped this into the Custom CSS.
border: 1px solid #DCDCDC !important;
background-color: #FFFFFF !important;
background-color: #DCDCDC !important;
I used this site for color references.
Seems as per picture attached we are not getting this code to stick.
I went in and cleared Theme Cache just to make sure and opened and closed the browser.
For giggles, I took the development site back to boost theme and cleared cache. checked to make sure it looked good.
then went back to Adaptable to check if we had the change, but sadly no.
Let me know I am entering this code wrong.