diff --git a/css/dark.css b/css/dark.css
index e69de29..aee4d4b 100644
--- a/css/dark.css
+++ b/css/dark.css
@@ -0,0 +1,6 @@
+
+body {
+    font-family: Arial, Helvetica, sans-serif;
+    color: #f5f5f5;
+    background-color: #555;
+}
diff --git a/css/light.css b/css/light.css
index dd1b8ec..97b6a48 100644
--- a/css/light.css
+++ b/css/light.css
@@ -1,4 +1,6 @@
 
 body {
     font-family: Arial, Helvetica, sans-serif;
+    color: #555;
+    background-color: whitesmoke;
 }
diff --git a/header.php b/header.php
index ecfaa1f..d204534 100644
--- a/header.php
+++ b/header.php
@@ -4,9 +4,9 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title><?php echo "$title"; ?></title>
-    <link rel="stylesheet" href="css/light.css">
+    <link rel="stylesheet" href="css/<?php echo "$theme"; ?>.css">
     <style>
-        body {
+        p.color {
             color: <?php echo "$color"; ?>;
         }
     </style>
diff --git a/index.php b/index.php
index 989a366..d1a5b2a 100644
--- a/index.php
+++ b/index.php
@@ -9,6 +9,13 @@ if (isset($_GET["cc"])) {
     $color = $_GET["cc"];
 }
 
+// změna theme stránky (dark/light)
+$theme = "light";
+if (isset($_GET["theme"])) {
+    // porušení pravidla - nikdy nepřiřazuj hodnotu přímo z GET (POST)
+    $theme = $_GET["theme"];
+}
+
 include "header.php";
 
 include "test.php";
diff --git a/paragraph.php b/paragraph.php
index 0aaf053..53d5c24 100644
--- a/paragraph.php
+++ b/paragraph.php
@@ -1,3 +1,7 @@
-<p>
+<p class="color">
     Lorem ipsum dolor sit amet, consectetur adipisicing elit. Cupiditate labore illum possimus consectetur tempora ipsa eos necessitatibus! Dolores eius qui officia ipsam dolorem iure voluptatum voluptas ut. Quas, qui recusandae!
-</p>
\ No newline at end of file
+</p>
+
+<p>
+    Necessitatibus illum animi ipsam pariatur, iure debitis commodi. Voluptatum illo officia, qui eum hic, ab rem culpa voluptatibus itaque earum deserunt doloremque?
+</p>
diff --git a/test.php b/test.php
index 347e12e..02f578b 100644
--- a/test.php
+++ b/test.php
@@ -5,4 +5,6 @@
    <a href="/?img=2">stránka s obrázkem 2</a>
    <a href="/?img=333">stránka s chybnou hodnotou obrázku</a>
    <a href="/?par=&cc=red">stránka s červeným textem</a>
+   <a href="/?par=&theme=dark">dark theme</a>
+   <a href="/?par=&theme=light">light theme</a>
 </p>