[theme] default to using default colors and, if default coloring is not enabled, turn -1 into white/black for fg/bg

src/lnav_config.hh

@@ -107,7 +107,7 @@ struct key_map {
 struct _lnav_config {
     std::string lc_ui_clock_format;
     bool lc_ui_dim_text;
-    bool lc_ui_default_colors;
+    bool lc_ui_default_colors{true};
     std::string lc_ui_keymap;
     std::string lc_ui_theme;
     std::unordered_map<std::string, key_map> lc_ui_keymaps;

src/view_curses.cc

@@ -619,6 +619,13 @@ inline attr_t attr_for_colors(int &pair_base, short fg, short bg)
             if (bg == COLOR_BLACK) {
                 bg = -1;
             }
+        } else {
+            if (fg == -1) {
+                fg = COLOR_WHITE;
+            }
+            if (bg == -1) {
+                bg = COLOR_BLACK;
+            }
         }
     }