Set Gray color as "Bright Black" (#1227)

## Description

Set Gray color as "Bright Black", judging from [Gentoo's documentation
about colors in terminal
emulators](https://wiki.gentoo.org/wiki/Terminal_emulator/Colors#Specifying_colors).

This has the nice effect of enabling custom theming of the gray color by
the terminal application (e.g., Konsole), so that the gray remains
readable with different themes.

Author: koutheir

gef.py

@@ -577,7 +577,7 @@ class Color:
     # \033      ->  Start an ANSI escape code for displaying colors
     colors = {
         "normal": "\001\033[0m\002",
-        "gray": "\001\033[1;38;5;240m\002",
+        "gray": "\001\033[90m\002",
         "light_gray": "\001\033[0;37m\002",
         "red": "\001\033[31m\002",
         "green": "\001\033[32m\002",