Change background color in mupdf from gray to black

--- a/platform/x11/x11_main.c   2019-11-12 01:50:05.059480869 -0500
+++ b/platform/x11/x11_main.c   2019-11-12 01:50:38.405480869 -0500
@@ -226,9 +226,9 @@
        xcwait = XCreateFontCursor(xdpy, XC_watch);
        xccaret = XCreateFontCursor(xdpy, XC_xterm);

-       xbgcolor.red = 0x7000;
-       xbgcolor.green = 0x7000;
-       xbgcolor.blue = 0x7000;
+       xbgcolor.red = 0x0000;
+       xbgcolor.green = 0x0000;
+       xbgcolor.blue = 0x0000;

        XAllocColor(xdpy, DefaultColormap(xdpy, xscr), &xbgcolor);

--- a/platform/gl/gl-main.c     2019-11-12 02:10:39.582480869 -0500
+++ b/platform/gl/gl-main.c     2019-11-12 02:10:56.281480869 -0500
@@ -1593,10 +1593,7 @@

 void run_main_loop(void)
 {
-       if (currentinvert)
-               glClearColor(0, 0, 0, 1);
-       else
-               glClearColor(0.3f, 0.3f, 0.3f, 1);
+       glClearColor(0, 0, 0, 1);
        ui_begin();
        fz_try(ctx)
        {