Sets the background color of selected text.
For supported color formats see change/read palette color.
TODO this color can not reliably restored to original state by reading this and later setting this to the read value. (at least vte; it has a special unset internal flag)
🛈 if the effective selection foreground and background colors exactly match xterm reverts to default colors. 🛈 This sequence does not change according to mode reverse display colors. Selection colors keep the same regardless of reverse setting and sequences for background/foreground don't switch.