UI: Use System theme volume meter colors

These comments do not actually match the color values here:
 * (0xff, 0xff, 0xff) is white
 * (0xcc, 0xcc, 0xcc) is light gray

Instead of just fixing the comments, use the values from the System
theme and also correct the comments.

UI/volume-control.cpp

@@ -853,8 +853,8 @@ VolumeMeter::VolumeMeter(QWidget *parent, obs_volmeter_t *obs_volmeter,
 
 	clipColor.setRgb(0xff, 0xff, 0xff);      // Bright white
 	magnitudeColor.setRgb(0x00, 0x00, 0x00); // Black
-	majorTickColor.setRgb(0xff, 0xff, 0xff); // Black
-	minorTickColor.setRgb(0xcc, 0xcc, 0xcc); // Black
+	majorTickColor.setRgb(0x00, 0x00, 0x00); // Black
+	minorTickColor.setRgb(0x32, 0x32, 0x32); // Dark gray
 	minimumLevel = -60.0;                    // -60 dB
 	warningLevel = -20.0;                    // -20 dB
 	errorLevel = -9.0;                       //  -9 dB