fix(agnoster): respect bg / fg user envs (#12897)

Co-authored-by: Carlo Sala <carlosalag@protonmail.com>

themes/agnoster.zsh-theme

@@ -36,12 +36,12 @@ CURRENT_BG='NONE'
 
 case ${SOLARIZED_THEME:-dark} in
     light)
-      CURRENT_FG='white'
-      CURRENT_DEFAULT_FG='white'
+      CURRENT_FG=${CURRENT_FG:-'white'}
+      CURRENT_DEFAULT_FG=${CURRENT_DEFAULT_FG:-'white'}
       ;;
     *)
-      CURRENT_FG='black'
-      CURRENT_DEFAULT_FG='default'
+      CURRENT_FG=${CURRENT_FG:-'black'}
+      CURRENT_DEFAULT_FG=${CURRENT_DEFAULT_FG:-'default'}
       ;;
 esac