update code snippet colors for dark theme

docs/prettify/threejs.css

-pre .str, code .str { color: #8000ff; } /* string */
+pre .str, code .str { color: #BB55FF; } /* string */
 pre .kwd, code .kwd { color: #30b030; } /* keyword */
-pre .com, code .com { color: #999999; } /* comment */
+pre .com, code .com { color: #666666; } /* comment */
 pre .typ, code .typ { color: #2194ce; } /* type */
-pre .lit, code .lit { color: #ff0080; } /* literal */
+pre .lit, code .lit { color: #ff3399; } /* literal */
 pre .pun, code .pun { color: #888888; } /* punctuation */
-pre .pln, code .pln { color: #444444; } /* plaintext */
+pre .pln, code .pln { color: #aaaaaa; } /* plaintext */
 pre .dec, code .dec { color: #22c0c4; } /* decimal */
 
 
 
 pre.prettyprint, code.prettyprint {
-	background-color: #F5F5F5;
+	background-color: #333333;
 	font-family: 'Roboto Mono', monospace;
 }