diff --git a/go/tw/tailwind.go b/go/tw/tailwind.go
index 533c83a8..66d844b6 100644
--- a/go/tw/tailwind.go
+++ b/go/tw/tailwind.go
@@ -1304,6 +1304,7 @@ func twCompile(input, baseDir string, candidates []string) (string, int, error)
 	var keyframes []*AstNode
 	var passthrough []*AstNode
 	var customUtilities []*AstNode
+	var customVariants []*AstNode
 	var safelistAdd []string
 	var safelistRemove []string
 	hasPreflight := false
@@ -1361,6 +1362,8 @@ func twCompile(input, baseDir string, candidates []string) (string, int, error)
 				processTheme(node)
 			case node.Kind == nAtRule && node.Name == "@utility":
 				customUtilities = append(customUtilities, node)
+			case node.Kind == nAtRule && node.Name == "@custom-variant":
+				customVariants = append(customVariants, node)
 			case node.Kind == nAtRule && node.Name == "@source":
 				literals, negate := parseSourceDirective(node.Params)
 				if negate {
@@ -1399,6 +1402,20 @@ func twCompile(input, baseDir string, candidates []string) (string, int, error)
 
 	ds := buildDesignSystem(theme)
 
+	// Register @custom-variant blocks. This is how a project defines `dark:` as a CLASS
+	// toggle rather than a media query — the built-in dark variant follows the OS, which
+	// a site with a theme switch cannot use:
+	//
+	//	@custom-variant dark (&:where(.dark, .dark *));
+	//
+	// Both of Tailwind's forms are accepted: the shorthand above, and the block form
+	// with an explicit @slot.
+	for _, cv := range customVariants {
+		if name, body, ok := parseCustomVariant(cv); ok {
+			ds.variants.fromAst(name, body, ds)
+		}
+	}
+
 	// Register @utility blocks as static utilities.
 	for _, u := range customUtilities {
 		name := strings.TrimSpace(u.Params)
@@ -8813,6 +8830,95 @@ func (v *Variants) kind(name string) variantKind {
 
 func (v *Variants) keys() []string { return v.order }
 
+// parseCustomVariant reads an @custom-variant at-rule into a name and the AST body that
+// fromAst expects (a body whose rules contain an @slot where the utility goes).
+//
+// Two forms, both from Tailwind:
+//
+//	@custom-variant dark (&:where(.dark, .dark *));   // shorthand
+//
+//	@custom-variant dark {                            // block, explicit slot
+//	    &:where(.dark, .dark *) { @slot; }
+//	}
+//
+// In the shorthand, a parenthesised selector starting with '@' is an at-rule
+// (`@custom-variant any-hover (@media (any-hover: hover))`), and anything else is a
+// selector. Several may be given, comma-separated at the top level.
+func parseCustomVariant(node *AstNode) (name string, body []*AstNode, ok bool) {
+	params := strings.TrimSpace(node.Params)
+	if params == "" {
+		return "", nil, false
+	}
+
+	// The name is the first token; whatever follows is the shorthand's parenthesised part.
+	i := strings.IndexAny(params, " \t(")
+	if i < 0 {
+		// No shorthand: it must be the block form, which carries its own @slot.
+		if len(node.Nodes) == 0 {
+			return "", nil, false
+		}
+		return params, node.Nodes, true
+	}
+	name = strings.TrimSpace(params[:i])
+	rest := strings.TrimSpace(params[i:])
+
+	if rest == "" {
+		if len(node.Nodes) == 0 {
+			return "", nil, false
+		}
+		return name, node.Nodes, true
+	}
+	if !strings.HasPrefix(rest, "(") || !strings.HasSuffix(rest, ")") {
+		return "", nil, false
+	}
+	inner := strings.TrimSpace(rest[1 : len(rest)-1])
+	if inner == "" {
+		return "", nil, false
+	}
+
+	for _, sel := range splitTopLevel(inner, ',') {
+		sel = strings.TrimSpace(sel)
+		if sel == "" {
+			continue
+		}
+		slot := atRule("@slot", "")
+		if strings.HasPrefix(sel, "@") {
+			// "@media (any-hover: hover)" -> name "@media", params "(any-hover: hover)"
+			at, params, _ := strings.Cut(sel, " ")
+			body = append(body, atRule(at, strings.TrimSpace(params), slot))
+			continue
+		}
+		body = append(body, styleRule(sel, slot))
+	}
+	if len(body) == 0 {
+		return "", nil, false
+	}
+	return name, body, true
+}
+
+// splitTopLevel splits on sep, ignoring separators nested inside brackets — a selector
+// list like `&:where(.dark, .dark *)` is ONE selector, and splitting it on its inner
+// comma would produce two broken halves.
+func splitTopLevel(s string, sep byte) []string {
+	var parts []string
+	depth := 0
+	start := 0
+	for i := 0; i < len(s); i++ {
+		switch s[i] {
+		case '(', '[':
+			depth++
+		case ')', ']':
+			depth--
+		case sep:
+			if depth == 0 {
+				parts = append(parts, s[start:i])
+				start = i + 1
+			}
+		}
+	}
+	return append(parts, s[start:])
+}
+
 func (v *Variants) compoundsWith(parent string, child *Variant) bool {
 	parentInfo, ok := v.variants[parent]
 	if !ok {