Yes, you can.
To put something inside a root, and also continue drilling, you can use .:, which means “here”.
So in your case it would be something like:
"more-info-light":
[...]
$:
# Put those styles in the shadowRoot element
.: |
div div.brightness{
background-color: blue !important;
}
# but also keep going into div div.color_temp...
"div div.color_temp":
"ha-labeled-slider":
$: |
ha-paper-slider{
background-image: -webkit-linear-gradient( right, rgb(255,121, 0) 0%, rgb(255,254, 250) 100% );
}