diff --git a/CHANGELOG.md b/CHANGELOG.md
index a293c721..e76f985b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -70,6 +70,7 @@
 ## Unreleased
 ### Added
 
+* TODO
 * `toplevel-tag` option (and `--toplevel-tag` command line options to
   `foot` and `footclient`), allowing you to set a custom toplevel
   tag. The compositor must implement the new `xdg-toplevel-tag-v1`
diff --git a/config.c b/config.c
index 515b088c..6c2d355d 100644
--- a/config.c
+++ b/config.c
@@ -25,6 +25,7 @@
 #include "input.h"
 #include "key-binding.h"
 #include "macros.h"
+#include "srgb.h"
 #include "tokenize.h"
 #include "util.h"
 #include "xmalloc.h"
@@ -3185,6 +3186,16 @@ parse_config_file(FILE *f, struct config *conf, const char *path, bool errors_ar
         errno = 0;
     }
 
+    struct color_theme *color_themes[] = {&conf->colors, &conf->colors2};
+    for (int i = 0; i < ALEN(color_themes); i++) {
+        struct color_theme *color_theme = color_themes[i];
+        if (color_theme->dim_blend_towards != DIM_BLEND_TOWARDS_INVALID)
+            continue;
+        color_theme->dim_blend_towards =
+            is_dark_theme(color_theme->fg, color_theme->bg)
+            ? DIM_BLEND_TOWARDS_BLACK : DIM_BLEND_TOWARDS_WHITE;
+    }
+
     if (errno != 0) {
         LOG_AND_NOTIFY_ERRNO("failed to read from configuration");
         if (errors_are_fatal)
@@ -3442,7 +3453,7 @@ config_load(struct config *conf, const char *conf_path,
             .flash_alpha = 0x7fff,
             .alpha = 0xffff,
             .alpha_mode = ALPHA_MODE_DEFAULT,
-            .dim_blend_towards = DIM_BLEND_TOWARDS_BLACK,
+            .dim_blend_towards = DIM_BLEND_TOWARDS_INVALID,
             .selection_fg = 0x80000000,  /* Use default bg */
             .selection_bg = 0x80000000,  /* Use default fg */
             .cursor = {
@@ -3538,7 +3549,6 @@ config_load(struct config *conf, const char *conf_path,
     memcpy(conf->colors.table, default_color_table, sizeof(default_color_table));
     memcpy(conf->colors.sixel, default_sixel_colors, sizeof(default_sixel_colors));
     memcpy(&conf->colors2, &conf->colors, sizeof(conf->colors));
-    conf->colors2.dim_blend_towards = DIM_BLEND_TOWARDS_WHITE;
 
     parse_modifiers(XKB_MOD_NAME_SHIFT, 5, &conf->mouse.selection_override_modifiers);
 
@@ -4096,6 +4106,30 @@ check_if_font_is_monospaced(const char *pattern,
     return is_monospaced;
 }
 
+pixman_color_t
+color_hex_to_pixman_srgb(uint32_t color, uint16_t alpha)
+{
+    return (pixman_color_t){
+        .alpha = alpha,  /* Consider alpha linear already? */
+        .red = srgb_decode_8_to_16((color >> 16) & 0xff),
+        .green = srgb_decode_8_to_16((color >> 8) & 0xff),
+        .blue = srgb_decode_8_to_16((color >> 0) & 0xff),
+    };
+}
+
+// See https://en.wikipedia.org/wiki/Relative_luminance
+static float relative_luminance(uint32_t rgb)
+{
+    pixman_color_t srgb =
+        color_hex_to_pixman_srgb(rgb, /*alpha=*/UINT16_MAX);
+    return 0.2126 * srgb.red + 0.7152 * srgb.green + 0.0722 * srgb.blue;
+}
+
+bool is_dark_theme(uint32_t fg, uint32_t bg)
+{
+    return relative_luminance(bg) < relative_luminance(fg);
+}
+
 #if 0
 xkb_mod_mask_t
 conf_modifiers_to_mask(const struct seat *seat,
diff --git a/config.h b/config.h
index fc5e290e..a4a01fac 100644
--- a/config.h
+++ b/config.h
@@ -148,6 +148,7 @@ struct color_theme {
     enum {
         DIM_BLEND_TOWARDS_BLACK,
         DIM_BLEND_TOWARDS_WHITE,
+        DIM_BLEND_TOWARDS_INVALID,
     } dim_blend_towards;
 
     enum {
@@ -473,3 +474,7 @@ conf_modifiers_to_mask(
 #endif
 bool check_if_font_is_monospaced(
     const char *pattern, user_notifications_t *notifications);
+
+pixman_color_t
+color_hex_to_pixman_srgb(uint32_t color, uint16_t alpha);
+bool is_dark_theme(uint32_t fg, uint32_t bg);
diff --git a/doc/foot-ctlseqs.7.scd b/doc/foot-ctlseqs.7.scd
index 40906ebf..86f36492 100644
--- a/doc/foot-ctlseqs.7.scd
+++ b/doc/foot-ctlseqs.7.scd
@@ -664,9 +664,7 @@ manipulation sequences. The generic format is:
 :  Query the current (color) theme mode
 :  contour
 :  The current color theme mode (light or dark) is reported as *CSI ?
-   997 ; 1|2 n*, where *1* means dark and *2* light. By convention, the
-   primary theme in foot is considered dark, and the alternative theme
-   light.
+   997 ; 1|2 n*, where *1* means dark and *2* light.
 
 
 # OSC
diff --git a/doc/foot.ini.5.scd b/doc/foot.ini.5.scd
index c9782895..20721056 100644
--- a/doc/foot.ini.5.scd
+++ b/doc/foot.ini.5.scd
@@ -1138,17 +1138,12 @@ dark theme (since the default theme is dark).
 # SECTION: colors2
 
 This section defines an alternative color theme. It has the exact same
-keys as the *colors* section. The default values are the same, except
-for *dim-blend-towards*, which defaults to *white* instead.
+keys and default values as the *colors* section.
 
 Note that values are not inherited. That is, if you set a value in
 *colors*, but not in *colors2*, the value from *colors* is not
 inherited by *colors2*.
 
-In the context of private mode 2031 (Dark and Light Mode detection),
-the alternative theme (i.e. the *colors2* section) is considered to be
-the light theme (since the default, the primary theme, is dark).
-
 # SECTION: csd
 
 This section controls the look of the _CSDs_ (Client Side
diff --git a/foot.ini b/foot.ini
index 2d170489..09950b64 100644
--- a/foot.ini
+++ b/foot.ini
@@ -132,7 +132,7 @@
 # bright7=ffffff   # bright white
 
 ## dimmed colors (see foot.ini(5) man page)
-# dim-blend-towards=black
+# dim-blend-towards=<black if dark mode else white>
 # dim0=<not set>
 # ...
 # dim7=<not-set>
@@ -171,8 +171,7 @@
 
 [colors2]
 # Alternative color theme, see man page foot.ini(5)
-# Same builtin defaults as [color], except for:
-# dim-blend-towards=white
+# Same builtin defaults as [color]
 
 [csd]
 # preferred=server
diff --git a/render.c b/render.c
index 1d0f08af..6e079016 100644
--- a/render.c
+++ b/render.c
@@ -40,7 +40,6 @@
 #include "selection.h"
 #include "shm.h"
 #include "sixel.h"
-#include "srgb.h"
 #include "url-mode.h"
 #include "util.h"
 #include "xmalloc.h"
@@ -229,17 +228,6 @@ attrs_to_font(const struct terminal *term, const struct attributes *attrs)
     return term->fonts[idx];
 }
 
-static pixman_color_t
-color_hex_to_pixman_srgb(uint32_t color, uint16_t alpha)
-{
-    return (pixman_color_t){
-        .alpha = alpha,  /* Consider alpha linear already? */
-        .red = srgb_decode_8_to_16((color >> 16) & 0xff),
-        .green = srgb_decode_8_to_16((color >> 8) & 0xff),
-        .blue = srgb_decode_8_to_16((color >> 0) & 0xff),
-    };
-}
-
 static inline pixman_color_t
 color_hex_to_pixman_with_alpha(uint32_t color, uint16_t alpha, bool srgb)
 {
diff --git a/subprojects/.wraplock b/subprojects/.wraplock
new file mode 100644
index 00000000..e69de29b
diff --git a/terminal.c b/terminal.c
index 3ef53daa..d24e03ca 100644
--- a/terminal.c
+++ b/terminal.c
@@ -1,4 +1,6 @@
 #include "terminal.h"
+#include "pixman.h"
+#include <stdint.h>
 
 #if defined(__GLIBC__)
 #include <malloc.h>
@@ -4785,10 +4787,6 @@ term_theme_toggle(struct terminal *term)
 /*
  * 1 - dark mode
  * 2 - light mode
- *
- * In foot, the themes aren't necessarily light/dark,
- * but by convention, the primary theme is dark, and
- * the alternative theme is light.
  */
 void term_send_color_theme_mode(struct terminal* term)
 {
@@ -4796,6 +4794,6 @@ void term_send_color_theme_mode(struct terminal* term)
         "\033[?997;2n",
         "\033[?997;1n",
     };
-    const bool is_dark = term->colors.active_theme == COLOR_THEME1;
+    const bool is_dark = is_dark_theme(term->colors.fg, term->colors.bg);
     term_to_slave(term, reply[is_dark], 9);
 }
diff --git a/tests/meson.build b/tests/meson.build
index 9baa064c..3d126973 100644
--- a/tests/meson.build
+++ b/tests/meson.build
@@ -1,6 +1,7 @@
 config_test = executable(
   'test-config',
   'test-config.c',
+  srgb_funcs,
   wl_proto_headers,
   link_with: [common, tokenize],
   dependencies: [pixman, xkb, fontconfig, wayland_client, fcft, tllist])