feat: default system theme (#419)

Co-authored-by: adamdottv <2363879+adamdottv@users.noreply.github.com>

packages/tui/internal/styles/background.go

@@ -1,6 +1,9 @@
 package styles
 
+import "image/color"
+
 type TerminalInfo struct {
+	Background       color.Color
 	BackgroundIsDark bool
 }
 
@@ -8,6 +11,7 @@ var Terminal *TerminalInfo
 
 func init() {
 	Terminal = &TerminalInfo{
+		Background:       color.Black,
 		BackgroundIsDark: true,
 	}
 }