update: use variables to define bg and fg

Xresources

@@ -1,17 +1,18 @@
-xterm*background: #0d2221
-xterm*foreground: lightgray
+#define bg #0d2221
+#define fg lightgray
 
+xterm*background: bg
+xterm*foreground: fg
 xterm*faceName: Fira Code VF
 xterm*visualBell: False
-
 xterm*VT100.Translations: #override \
 		Ctrl Shift <Key>V:	insert-selection(CLIPBOARD) \n\
 		Ctrl Shift <Key>C:	copy-selection(CLIPBOARD)
 
 URxvt*font: xft:DejaVu Sans Mono: size=10
 URxvt*boldFont: xft:DejaVu Sans Mono: size=10
-URxvt*background: #0d2221
-URxvt*foreground: lightgray
+URxvt*background: bg
+URxvt*foreground: fg
 URxvt*scrollBar: false
 URxvt.iso14755: false
 URxvt.iso14755_52: false