diff --git a/cl-tui.asd b/cl-tui.asd
index 3ccc036..86b7784 100644
--- a/cl-tui.asd
+++ b/cl-tui.asd
@@ -21,7 +21,8 @@
      (:file "dirty")
      (:file "box" :depends-on ("package"))
      (:file "text" :depends-on ("package" "box"))
-     (:file "render" :depends-on ("package" "box" "text")))))
+     (:file "render" :depends-on ("package" "box" "text"))
+     (:file "theme" :depends-on ("package")))))
   :in-order-to ((test-op (test-op :cl-tui-tests))))
 
 (asdf:defsystem :cl-tui-tests
@@ -38,6 +39,7 @@
     :components
     ((:file "box-tests")
      (:file "dirty-tests")
-     (:file "render-tests"))))
+     (:file "render-tests")
+     (:file "theme-tests"))))
   :perform (test-op (o c)
              (uiop:symbol-call :cl-tui-backend-test '#:run-tests)))
diff --git a/demo.lisp b/demo.lisp
new file mode 100644
index 0000000..f373266
--- /dev/null
+++ b/demo.lisp
@@ -0,0 +1,28 @@
+;; demo.lisp — minimal cl-tui demo
+(load "/root/quicklisp/setup.lisp")
+(ql:quickload :fiveam :silent t)
+(load "backend/package.lisp")
+(load "backend/classes.lisp")
+(load "backend/simple.lisp")
+(load "backend/modern.lisp")
+(load "layout/layout.lisp")
+(load "src/components/package.lisp")
+(load "src/components/dirty.lisp")
+(load "src/components/box.lisp")
+(load "src/components/text.lisp")
+(load "src/components/render.lisp")
+(in-package :cl-tui.box)
+
+;; Demo 1: Simple backend (ASCII)
+(let* ((b (make-simple-backend))
+       (bx (make-box :border-style :rounded :title " Hello World " :width 30 :height 5)))
+  (compute-layout (box-layout-node bx) 30 5)
+  (render bx b))
+
+;; Demo 2: Box with text inside
+(let* ((b (make-simple-backend))
+       (tx (make-text "This is cl-tui in action!" :width 28 :height 1)))
+  (setf (layout-node-direction (text-layout-node tx)) :column)
+  (compute-layout (text-layout-node tx) 28 1)
+  (render tx b)
+  (format t "~%~%"))
diff --git a/src/components/package.lisp b/src/components/package.lisp
index 34cdfd3..3722403 100644
--- a/src/components/package.lisp
+++ b/src/components/package.lisp
@@ -24,5 +24,8 @@
    #:render #:render-screen #:render-node
    #:component-layout-node #:component-children #:component-parent
    #:available-width #:available-height
-   #:propagate-dirty))
+   #:propagate-dirty
+   ;; Theme engine
+   #:theme #:make-theme #:theme-mode
+   #:theme-color #:load-preset #:define-preset))
 (in-package :cl-tui.box)
diff --git a/src/components/theme-tests.lisp b/src/components/theme-tests.lisp
new file mode 100644
index 0000000..b342719
--- /dev/null
+++ b/src/components/theme-tests.lisp
@@ -0,0 +1,61 @@
+(in-package :cl-tui-box-test)
+(in-suite box-suite)
+
+(test theme-create-default
+  "A theme can be created with default mode"
+  (let ((th (make-theme)))
+    (is (typep th 'theme))
+    (is (eql (theme-mode th) :dark))))
+
+(test theme-create-light
+  "A theme can be created in light mode"
+  (let ((th (make-theme :mode :light)))
+    (is (eql (theme-mode th) :light))))
+
+(test theme-color-set-and-get
+  "theme-color setf/get works"
+  (let ((th (make-theme)))
+    (setf (theme-color th :primary) "#FFD700")
+    (is (string= (theme-color th :primary) "#FFD700"))))
+
+(test theme-color-unknown-returns-nil
+  "Unknown roles return nil"
+  (let ((th (make-theme)))
+    (is (null (theme-color th :nonexistent)))))
+
+(test load-default-dark-preset
+  "Loading the default dark preset populates roles"
+  (let ((th (make-theme :mode :dark)))
+    (load-preset th :default)
+    (is (string= (theme-color th :primary) "#FFD700"))
+    (is (string= (theme-color th :background) "#1A1A2E"))
+    (is (string= (theme-color th :error) "#FF4444"))))
+
+(test load-default-light-preset
+  "Light variant has different colors"
+  (let ((th (make-theme :mode :light)))
+    (load-preset th :default)
+    (is (string= (theme-color th :primary) "#B8860B"))
+    (is (string= (theme-color th :background) "#F8F9FA"))))
+
+(test load-nord-preset
+  "Nord preset has different colors than default"
+  (let ((th (make-theme :mode :dark)))
+    (load-preset th :nord)
+    (is (string= (theme-color th :primary) "#88C0D0"))
+    (is (string= (theme-color th :background) "#2E3440"))))
+
+(test load-preset-unknown-warns
+  "Unknown preset warns but doesn't error"
+  (let ((th (make-theme)))
+    (load-preset th :nonexistent)
+    (is (null (theme-color th :primary)))))
+
+(test preset-switch-mode
+  "Switching mode and reloading changes colors"
+  (let ((th (make-theme :mode :dark)))
+    (load-preset th :default)
+    (is (string= (theme-color th :background) "#1A1A2E"))
+    (setf (theme-mode th) :light)
+    (load-preset th :default)
+    (is (string= (theme-color th :background) "#F8F9FA"))))
diff --git a/src/components/theme.lisp b/src/components/theme.lisp
new file mode 100644
index 0000000..a2c3f45
--- /dev/null
+++ b/src/components/theme.lisp
@@ -0,0 +1,86 @@
+(in-package :cl-tui.box)
+
+;; ── Theme Engine ──────────────────────────────────────────────
+
+(defclass theme ()
+  ((mode :initform :dark :initarg :mode :accessor theme-mode)
+   (roles :initform (make-hash-table) :accessor theme-roles)))
+
+(defun make-theme (&key (mode :dark))
+  (make-instance 'theme :mode mode))
+
+(defun theme-color (theme role)
+  "Resolve a semantic ROLE to a hex color string in THEME."
+  (gethash role (theme-roles theme)))
+
+(defun (setf theme-color) (hex theme role)
+  "Set the hex color for a semantic ROLE in THEME."
+  (setf (gethash role (theme-roles theme)) hex))
+
+(defparameter *presets* (make-hash-table :test #'eq))
+
+(defmacro define-preset (name &key dark light)
+  "Define a theme preset with DARK and LIGHT variants.
+NAME should be a keyword (e.g., :default, :nord)."
+  `(setf (gethash ,name *presets*) '(:dark ,dark :light ,light)))
+
+(defun load-preset (theme preset-name)
+  "Load PRESET-NAME (a keyword) into THEME, overwriting role mappings."
+  (let ((preset (gethash preset-name *presets*)))
+    (if preset
+        (let* ((variant (if (eql (theme-mode theme) :dark)
+                            (getf preset :dark)
+                            (getf preset :light)))
+               (roles (theme-roles theme)))
+          (clrhash roles)
+          (loop for (role hex) on variant by #'cddr
+                do (setf (gethash role roles) hex)))
+        (warn "Unknown preset: ~S" preset-name))))
+
+(define-preset :default
+  :dark  (:primary "#FFD700" :secondary "#B8860B" :accent "#FFA500"
+          :error "#FF4444" :warning "#FF8800" :success "#44BB44" :info "#4488FF"
+          :text "#FFFFFF" :text-muted "#888888"
+          :background "#1A1A2E" :background-panel "#16213E" :background-element "#0F3460"
+          :border "#334155" :border-active "#FFD700"
+          :diff-added "#164B16" :diff-removed "#4B1616" :diff-context "#1A1A2E"
+          :markdown-heading "#FFD700" :markdown-code "#334155"
+          :markdown-link "#4488FF" :markdown-quote "#888888"
+          :syntax-keyword "#FF79C6" :syntax-function "#50FA7B"
+          :syntax-string "#F1FA8C" :syntax-number "#BD93F9"
+          :syntax-comment "#6272A4" :syntax-type "#8BE9FD")
+  :light (:primary "#B8860B" :secondary "#8B6914" :accent "#FF8C00"
+          :error "#CC0000" :warning "#CC6600" :success "#228B22" :info "#0055CC"
+          :text "#1A1A2E" :text-muted "#888888"
+          :background "#F8F9FA" :background-panel "#FFFFFF" :background-element "#E9ECEF"
+          :border "#DEE2E6" :border-active "#B8860B"
+          :diff-added "#DFD" :diff-removed "#FDD" :diff-context "#F8F9FA"
+          :markdown-heading "#B8860B" :markdown-code "#E9ECEF"
+          :markdown-link "#0055CC" :markdown-quote "#888888"
+          :syntax-keyword "#D63384" :syntax-function "#198754"
+          :syntax-string "#FFC107" :syntax-number "#6F42C1"
+          :syntax-comment "#6C757D" :syntax-type "#0DCAF0"))
+
+(define-preset :nord
+  :dark  (:primary "#88C0D0" :secondary "#81A1C1" :accent "#5E81AC"
+          :error "#BF616A" :warning "#D08770" :success "#A3BE8C" :info "#B48EAD"
+          :text "#ECEFF4" :text-muted "#616E88"
+          :background "#2E3440" :background-panel "#3B4252" :background-element "#434C5E"
+          :border "#4C566A" :border-active "#88C0D0"
+          :diff-added "#164B16" :diff-removed "#4B1616" :diff-context "#2E3440"
+          :markdown-heading "#88C0D0" :markdown-code "#3B4252"
+          :markdown-link "#81A1C1" :markdown-quote "#616E88"
+          :syntax-keyword "#81A1C1" :syntax-function "#A3BE8C"
+          :syntax-string "#EBCB8B" :syntax-number "#B48EAD"
+          :syntax-comment "#616E88" :syntax-type "#88C0D0")
+  :light (:primary "#5E81AC" :secondary "#81A1C1" :accent "#88C0D0"
+          :error "#BF616A" :warning "#D08770" :success "#A3BE8C" :info "#B48EAD"
+          :text "#2E3440" :text-muted "#8F9BB3"
+          :background "#ECEFF4" :background-panel "#FFFFFF" :background-element "#E5E9F0"
+          :border "#D8DEE9" :border-active "#5E81AC"
+          :diff-added "#DFD" :diff-removed "#FDD" :diff-context "#ECEFF4"
+          :markdown-heading "#5E81AC" :markdown-code "#E5E9F0"
+          :markdown-link "#81A1C1" :markdown-quote "#8F9BB3"
+          :syntax-keyword "#81A1C1" :syntax-function "#A3BE8C"
+          :syntax-string "#D08770" :syntax-number "#B48EAD"
+          :syntax-comment "#8F9BB3" :syntax-type "#88C0D0"))