Add darker colors for light terminal backgrounds (merbanan#2864)

src/term_ctl.c

@@ -223,6 +223,38 @@ int term_get_columns(void *ctx)
 #endif
 }
 
+/**
+Returns wether the environment suggests a dark or light
+terminal background.
+
+Our default color theme is for dark backgrounds.
+
+Check the COLORFGBG environment variable, which should
+be 15;0 for a dark theme and 0;15 for a light theme.
+
+If the last value is 7 or 9 to 15 then assume a light theme.
+
+@return 1 if a light background was detected, 0 for a dark background otherwise
+*/
+static int term_get_bg(void)
+{
+    char const *colorfgbg = getenv("COLORFGBG");
+    if (!colorfgbg) {
+        return 0; // default dark theme
+    }
+    char *p = strrchr(colorfgbg, ';');
+    if (!p) {
+        return 0; // default dark theme
+    }
+
+    // Check if the last value is 7 or 9 to 15.
+    if (p[1] == '7' || p[1] == '9'
+            || (p[1] == '1' && p[2] != '\0')) {
+        return 1; // light theme
+    }
+    return 0; // dark theme
+}
+
 int term_has_color(void *ctx)
 {
 #ifdef _WIN32
@@ -281,6 +313,12 @@ void term_ring_bell(void *ctx)
 
 void term_set_fg(void *ctx, term_color_t color)
 {
+    // Cache the detected terminal background color
+    static int light_bg = -1;
+    if (light_bg == -1) {
+        light_bg = term_get_bg();
+    }
+
 #ifdef _WIN32
     console_t *console = (console_t *)ctx;
     if (!console->ansi) {
@@ -291,14 +329,28 @@ void term_set_fg(void *ctx, term_color_t color)
 #else
     FILE *fp = (FILE *)ctx;
 #endif
-    if (color == TERM_COLOR_RESET)
+    if (color == TERM_COLOR_RESET) {
         fprintf(fp, "\033[0m");
-    else
-        fprintf(fp, "\033[%d;1m", color);
+    }
+    else if (light_bg) {
+        fprintf(fp, "\033[%dm", color); // normal colors on light backgrounds
+    }
+    else {
+        fprintf(fp, "\033[%d;1m", color); // bright/bold colors on dark backgrounds
+    }
 }
 
 void term_set_bg(void *ctx, term_color_t bg, term_color_t fg)
 {
+    // Cache the detected terminal background color
+    static int light_bg = -1;
+    if (light_bg == -1) {
+        light_bg = term_get_bg();
+    }
+    if (light_bg && fg >= TERM_COLOR_BRIGHT_BLACK && fg <= TERM_COLOR_BRIGHT_WHITE) {
+        fg -= 60; // remove bright/bold foreground on light backgrounds
+    }
+
     if (bg < TERM_COLOR_BLACK
             || (bg > TERM_COLOR_WHITE && bg < TERM_COLOR_BRIGHT_BLACK)
             || bg > TERM_COLOR_BRIGHT_WHITE) {