UI: support dark mode in about window

Change-Id: Ieaefa2b8c7d2190c2462da3dd1abd0965394d6c0

Credits.rtf

 {\rtf1\ansi\ansicpg1252\cocoartf1671\cocoasubrtf100
 {\fonttbl\f0\fswiss\fcharset0 Helvetica-Oblique;\f1\fswiss\fcharset0 Helvetica-BoldOblique;\f2\fswiss\fcharset0 Helvetica-Bold;
 }
-{\colortbl;\red255\green255\blue255;}
-{\*\expandedcolortbl;;}
+{\colortbl;\red255\green255\blue255;\red0\green0\blue0;}
+{\*\expandedcolortbl;;\cssrgb\c0\c0\c0\cname textColor;}
 \margl1440\margr1440\vieww9000\viewh8400\viewkind0
 \pard\tx566\tx1133\tx1700\tx2267\tx2834\tx3401\tx3968\tx4535\tx5102\tx5669\tx6236\tx6803\pardirnatural\qc\partightenfactor0
 
-\f0\i\fs24 \cf0 \
+\f0\i\fs24 \cf2 \
 
 \fs28 "
 \f1\b Free as in Freedom"
 \f0\b0\fs24 \
 \
 \pard\tx566\tx1133\tx1700\tx2267\tx2834\tx3401\tx3968\tx4535\tx5102\tx5669\tx6236\tx6803\pardirnatural\qc\partightenfactor0
-{\field{\*\fldinst{HYPERLINK "https://jami.net/"}}{\fldrslt \cf0 jami.net}}\
+{\field{\*\fldinst{HYPERLINK "https://jami.net/"}}{\fldrslt \cf2
+    jami.net}}\
 \
 \pard\tx566\tx1133\tx1700\tx2267\tx2834\tx3401\tx3968\tx4535\tx5102\tx5669\tx6236\tx6803\pardirnatural\qc\partightenfactor0
 
-\f2\i0\b \cf0 Authors
+\f2\i0\b \cf2 Authors
 \f0\i\b0 \
 Adrien B\'e9raud\
 Alexandr Sergheev\
@@ -62,5 +63,5 @@ Based on SFLphone's project\
 \f2\i0\b Translated by
 \f0\i\b0 \
 \pard\tx566\tx1133\tx1700\tx2267\tx2834\tx3401\tx3968\tx4535\tx5102\tx5669\tx6236\tx6803\pardirnatural\qc\partightenfactor0
-{\field{\*\fldinst{HYPERLINK "https://www.transifex.com/savoirfairelinux/jami/"}}{\fldrslt \cf0 https://www.transifex.com/savoirfairelinux/jami/}}\
+{\field{\*\fldinst{HYPERLINK "https://www.transifex.com/savoirfairelinux/jami/"}}{\fldrslt \cf2 https://www.transifex.com/savoirfairelinux/jami/}}\
 }