fix windows menuconfig style support

1. back is not the color name in _NAMED_COLORS
2. windows terminal python enviroment not support the -1 as the color

Author: eastWillow

tools/kconfig/menuconfig.py

@@ -285,7 +285,7 @@
 
 _STYLES = {
     "default": """
-    path=fg:white,bg:back,bold
+    path=fg:white,bg:black,bold
     separator=fg:white,bg:blue,bold
     list=fg:white,bg:black
     selection=fg:black,bg:white,bold