If you would like to have upper or lower nibble stand out, you can add this to your theme file and configure the nibbles’ color to compliment each other.
<dict>
<key>name</key>
<string>Upper Byte Nibble</string>
<key>scope</key>
<string>raw.nibble.upper</string>
<key>settings</key>
<dict>
<key>foreground</key>
<string>#66D9EF</string>
</dict>
</dict>
<dict>
<key>name</key>
<string>Lower Byte Nibble</string>
<key>scope</key>
<string>raw.nibble.lower</string>
<key>settings</key>
<dict>
<key>foreground</key>
<string>#FFFFFF</string>
</dict>
</dict>
I am purposely not scoping this by default because certain themes might not have good contrast colors for whatever I scope them as. This way, you can add it if you want it and have complete control to make sure they stand out how you like them.