Detect light/dark theme when using legacy conhost.

Can't work in Windows Terminal until Terminal#10639 is fixed.
microsoft/terminal#10639

clink/app/src/host/host.cpp

@@ -872,6 +872,13 @@ bool host::edit_line(const char* prompt, const char* rprompt, str_base& out, boo
         lua.load_scripts();
     }
 
+    // Detect light vs dark console theme.
+    if (send_event)
+    {
+        extern void detect_console_default_attr();
+        detect_console_default_attr();
+    }
+
     // Send oninject event; one time only.
     static bool s_injected = false;
     if (send_event && !s_injected)

clink/lua/src/console_api.cpp

@@ -31,6 +31,20 @@ static SHORT GetConsoleNumLines(const CONSOLE_SCREEN_BUFFER_INFO& csbi)
     return bottom_Y + 1;
 }
 
+//------------------------------------------------------------------------------
+static uint16 s_default_attr = 0x07;
+void detect_console_default_attr()
+{
+    s_default_attr = 0x07;
+
+    CONSOLE_SCREEN_BUFFER_INFO csbiInfo;
+    HANDLE h = GetStdHandle(STD_OUTPUT_HANDLE);
+    if (!GetConsoleScreenBufferInfo(h, &csbiInfo))
+        return;
+
+    s_default_attr = csbiInfo.wAttributes;
+}
+
 //------------------------------------------------------------------------------
 class input_scope
 {
@@ -813,6 +827,22 @@ static int32 get_color_table(lua_State* state)
         lua_rawset(state, -3);
     }
 
+    // Luminance range is 0..3000 inclusive.
+    #define luminance(cr) \
+        (((299 * DWORD(GetRValue(cr))) + \
+          (587 * DWORD(GetGValue(cr))) + \
+          (114 * DWORD(GetBValue(cr)))) / 85)
+
+    const uint32 lum_bg = luminance(csbix.ColorTable[(s_default_attr & 0xf0) >> 4]);
+    if (lum_bg > luminance(RGB(128, 128, 128)))
+    {
+        lua_pushliteral(state, "light");
+        lua_pushboolean(state, true);
+        lua_rawset(state, -3);
+    }
+
+    #undef luminance
+
     return 1;
 }