diff --git a/hermes_cli/web_server.py b/hermes_cli/web_server.py
index 3ba9c1926..6f0df5e2c 100644
--- a/hermes_cli/web_server.py
+++ b/hermes_cli/web_server.py
@@ -16304,6 +16304,64 @@ def _normalise_prefix(raw: Optional[str]) -> str:
     return normalise_prefix(raw)
 
 
+def _render_active_theme_bootstrap_css() -> str:
+    """Critical-CSS shim for the active user theme.
+
+    Returns a ``<style>`` block with the ``:root`` CSS variables that
+    ``ThemeProvider.applyTheme()`` installs once the
+    ``/api/dashboard/themes`` round-trip completes.  The goal is to
+    eliminate the green flash where the first paint shows the bundle's
+    default Hermes Teal canvas before the SPA flips the configured user
+    theme into place.
+
+    Built-in themes return an empty string — their full definitions live
+    in ``web/src/themes/presets.ts`` and are applied by the bundle
+    before paint, so no shim is needed for them.
+    """
+    try:
+        config = load_config()
+        active = cfg_get(config, "dashboard", "theme", default="default")
+        if not active or not isinstance(active, str):
+            return ""
+        # Built-in: the bundle already owns the definition, no flash.
+        if any(b["name"] == active for b in _BUILTIN_DASHBOARD_THEMES):
+            return ""
+        for theme in _discover_user_themes():
+            if theme.get("name") != active:
+                continue
+            palette = theme.get("palette") or {}
+            bg = palette.get("background") or {}
+            mg = palette.get("midground") or {}
+            bg_hex = bg.get("hex", "#0a0a0a") if isinstance(bg, dict) else "#0a0a0a"
+            mg_hex = mg.get("hex", "#e5e5e5") if isinstance(mg, dict) else "#e5e5e5"
+            typo = theme.get("typography") or {}
+            font_sans = typo.get("fontSans") or _THEME_DEFAULT_TYPOGRAPHY["fontSans"]
+            base_size = typo.get("baseSize") or _THEME_DEFAULT_TYPOGRAPHY["baseSize"]
+            # Defensive ``</style>`` escape — current values are well-known
+            # hex/font strings, but this keeps the helper safe if it is
+            # later extended to ship user-authored CSS literals.
+            def _esc(s: str) -> str:
+                return str(s).replace("</", "<\\/")
+            return (
+                '<style id="hermes-theme-bootstrap">'
+                ":root{"
+                f"--background-base:{_esc(bg_hex)};"
+                f"--color-background:{_esc(bg_hex)};"
+                f"--midground-base:{_esc(mg_hex)};"
+                f"--color-midground:{_esc(mg_hex)};"
+                f"--font-sans:{_esc(font_sans)};"
+                f"--font-base-size:{_esc(base_size)};"
+                "}"
+                f"html,body{{background-color:{_esc(bg_hex)};color:{_esc(mg_hex)};"
+                f"font-family:{_esc(font_sans)};font-size:{_esc(base_size)};}}"
+                "</style>"
+            )
+        return ""
+    except Exception:
+        _log.debug("theme bootstrap render failed", exc_info=True)
+        return ""
+
+
 def mount_spa(application: FastAPI):
     """Mount the built SPA. Falls back to index.html for client-side routing.
 
@@ -16378,6 +16436,16 @@ def mount_spa(application: FastAPI):
             html = html.replace('href="/fonts/', f'href="{prefix}/fonts/')
             html = html.replace('href="/ds-assets/', f'href="{prefix}/ds-assets/')
             html = html.replace('src="/ds-assets/', f'src="{prefix}/ds-assets/')
+        # Theme flash mitigation: when the active theme is a user theme
+        # (``HERMES_HOME/dashboard-themes/<name>.yaml``), inject a minimal
+        # critical-CSS block so the first paint uses the target palette.
+        # Without this the SPA paints the default Hermes Teal canvas, then
+        # ``ThemeProvider`` flips the CSS variables once
+        # ``/api/dashboard/themes`` resolves.  Built-in themes are already
+        # in the bundle's ``presets.ts`` so no shim is needed for them.
+        theme_bootstrap = _render_active_theme_bootstrap_css()
+        if theme_bootstrap:
+            html = html.replace("</head>", f"{theme_bootstrap}</head>", 1)
         html = html.replace("</head>", f"{bootstrap_script}</head>", 1)
         return HTMLResponse(
             html,