Add dark-bg color scheme + enable colors by default if !NO_COLOR

The dark-bg scheme draws an explicit black background, so should also be
readable in terminals with a light background. That's been the main
show-stopper for enabling colors by default, as the 'dark' color scheme
is totally unreadable on light backgrounds and there's no way to detect
what background the terminal is actually using.

doc/ncdu.pod

@@ -136,9 +136,12 @@ accidentally press 'q' during or after a very long scan.
 
 =item --color I<SCHEME>
 
-Select a color scheme. Currently only two schemes are recognized: I<off> to
-disable colors (the default) and I<dark> for a color scheme intended for dark
-backgrounds.
+Select a color scheme. The following schemes are recognized: I<off> to disable
+colors, I<dark> for a color scheme intended for dark backgrounds and I<dark-bg>
+for a variation of the I<dark> color scheme that also works in terminals with a
+light background.
+
+The default is I<dark-bg> unless the C<NO_COLOR> environment variable is set.
 
 =back
 

src/main.c

@@ -122,6 +122,8 @@ static void argv_parse(int argc, char **argv) {
   char *import = NULL;
   char *dir = NULL;
 
+  uic_theme = getenv("NO_COLOR") ? 0 : 2;
+
   static yopt_opt_t opts[] = {
     { 'h', 0, "-h,-?,--help" },
     { 'q', 0, "-q" },
@@ -177,7 +179,7 @@ static void argv_parse(int argc, char **argv) {
       printf("  --exclude-firmlinks        Exclude firmlinks on macOS\n");
 #endif
       printf("  --confirm-quit             Confirm quitting ncdu\n");
-      printf("  --color SCHEME             Set color scheme (off/dark)\n");
+      printf("  --color SCHEME             Set color scheme (off/dark/dark-bg)\n");
       exit(0);
     case 'q': update_delay = 2000; break;
     case 'v':
@@ -229,6 +231,7 @@ static void argv_parse(int argc, char **argv) {
     case 'c':
       if(strcmp(val, "off") == 0)  { uic_theme = 0; }
       else if(strcmp(val, "dark") == 0) { uic_theme = 1; }
+      else if(strcmp(val, "dark-bg") == 0) { uic_theme = 2; }
       else {
         fprintf(stderr, "Unknown --color option: %s\n", val);
         exit(1);

src/util.c

@@ -250,10 +250,16 @@ static const struct {
   short fg, bg;
   int attr;
 } color_defs[] = {
-#define C(name, off_fg, off_bg, off_a, dark_fg, dark_bg, dark_a) \
-  {off_fg,  off_bg,  off_a}, \
-  {dark_fg, dark_bg, dark_a},
+#define COLOR__ -1
+#define B A_BOLD
+#define R A_REVERSE
+#define C(name, off_fg, off_bg, off_a, dark_fg, dark_bg, dark_a, darkbg_fg, darkbg_bg, darkbg_a) \
+  {COLOR_##off_fg,    COLOR_##off_bg,    off_a}, \
+  {COLOR_##dark_fg,   COLOR_##dark_bg,   dark_a}, \
+  {COLOR_##darkbg_fg, COLOR_##darkbg_bg, darkbg_a},
   UI_COLORS
+#undef B
+#undef R
 #undef C
   {0,0,0}
 };
@@ -264,7 +270,7 @@ void uic_init() {
   start_color();
   use_default_colors();
   for(i=0; i<sizeof(colors)/sizeof(*colors)-1; i++) {
-    j = i*2 + uic_theme;
+    j = i*3 + uic_theme;
     init_pair(i+1, color_defs[j].fg, color_defs[j].bg);
     colors[i] = color_defs[j].attr | COLOR_PAIR(i+1);
   }

src/util.h

@@ -31,24 +31,23 @@
 
 
 /* UI colors: (foreground, background, attrs)
- *  NAME         OFF                        DARK
- */
+ *  NAME         OFF         DARK                DARK-BG   */
 #define UI_COLORS \
-  C(DEFAULT,     -1,-1,0               ,    -1,           -1,         0     )\
-  C(BOX_TITLE,   -1,-1,A_BOLD          ,    COLOR_BLUE,   -1,         A_BOLD)\
-  C(HD,          -1,-1,A_REVERSE       ,    COLOR_BLACK,  COLOR_CYAN, 0     )    /* header & footer */\
-  C(SEL,         -1,-1,A_REVERSE       ,    COLOR_WHITE,  COLOR_GREEN,A_BOLD)\
-  C(NUM,         -1,-1,0               ,    COLOR_YELLOW, -1,         A_BOLD)\
-  C(NUM_HD,      -1,-1,A_REVERSE       ,    COLOR_YELLOW, COLOR_CYAN, A_BOLD)\
-  C(NUM_SEL,     -1,-1,A_REVERSE       ,    COLOR_YELLOW, COLOR_GREEN,A_BOLD)\
-  C(KEY,         -1,-1,A_BOLD          ,    COLOR_YELLOW, -1,         A_BOLD)\
-  C(KEY_HD,      -1,-1,A_BOLD|A_REVERSE,    COLOR_YELLOW, COLOR_CYAN, A_BOLD)\
-  C(DIR,         -1,-1,0               ,    COLOR_BLUE,   -1,         A_BOLD)\
-  C(DIR_SEL,     -1,-1,A_REVERSE       ,    COLOR_BLUE,   COLOR_GREEN,A_BOLD)\
-  C(FLAG,        -1,-1,0               ,    COLOR_RED,    -1,         0     )\
-  C(FLAG_SEL,    -1,-1,A_REVERSE       ,    COLOR_RED,    COLOR_GREEN,0     )\
-  C(GRAPH,       -1,-1,0               ,    COLOR_MAGENTA,-1,         0     )\
-  C(GRAPH_SEL,   -1,-1,A_REVERSE       ,    COLOR_MAGENTA,COLOR_GREEN,0     )
+  C(DEFAULT,     _,_,0  ,    _,      _,    0,    WHITE,  BLACK,0)\
+  C(BOX_TITLE,   _,_,B  ,    BLUE,   _,    B,    BLUE,   BLACK,B)\
+  C(HD,          _,_,R  ,    BLACK,  CYAN, 0,    BLACK,  CYAN, 0)    /* header & footer */\
+  C(SEL,         _,_,R  ,    WHITE,  GREEN,B,    WHITE,  GREEN,B)\
+  C(NUM,         _,_,0  ,    YELLOW, _,    B,    YELLOW, BLACK,B)\
+  C(NUM_HD,      _,_,R  ,    YELLOW, CYAN, B,    YELLOW, CYAN, B)\
+  C(NUM_SEL,     _,_,R  ,    YELLOW, GREEN,B,    YELLOW, GREEN,B)\
+  C(KEY,         _,_,B  ,    YELLOW, _,    B,    YELLOW, BLACK,B)\
+  C(KEY_HD,      _,_,B|R,    YELLOW, CYAN, B,    YELLOW, CYAN, B)\
+  C(DIR,         _,_,0  ,    BLUE,   _,    B,    BLUE,   BLACK,B)\
+  C(DIR_SEL,     _,_,R  ,    BLUE,   GREEN,B,    BLUE,   GREEN,B)\
+  C(FLAG,        _,_,0  ,    RED,    _,    0,    RED,    BLACK,0)\
+  C(FLAG_SEL,    _,_,R  ,    RED,    GREEN,0,    RED,    GREEN,0)\
+  C(GRAPH,       _,_,0  ,    MAGENTA,_,    0,    MAGENTA,BLACK,0)\
+  C(GRAPH_SEL,   _,_,R  ,    MAGENTA,GREEN,0,    MAGENTA,GREEN,0)
 
 enum ui_coltype {
 #define C(name, ...) UIC_##name,