I found this on a CSS site and tweaked it a little for Retool and the results are gorgeous 
main {
background: linear-gradient(-45deg, #ee7752, {{ theme.primary }}, #23a6d5, {{ theme.secondary }});
background-size: 400% 400%;
animation: gradient 30s ease infinite;
height: 100vh;
}
@keyframes gradient {
0% {
background-position: 0% 50%;
}
50% {
background-position: 100% 50%;
}
100% {
background-position: 0% 50%;
}
}
It slowly cycles through the gradients while following the theme's coloring. I have two static values, but you could use all theme values if you wanted.