passei as cores com as var e tambem fiz as alterações que a Caroline indicou.

sing.php

@@ -48,19 +48,12 @@
                     <input type="password" id="registerPassword" name="password" placeholder=" ">
                     <label for="registerPassword">Senha</label>
                 </div>
-                <button type="submit" class="register-button" id="registerBtn">Registrar</button>
+                <button type="submit" class="register-button btn" id="registerBtn">Registrar</button>
             </form>
             <button class="btn" id="showLoginBtn">Voltar para Login</button>
         </div>
     </div>
 
-    <div id="custom-alert" class="custom-alert-hidden">
-        <div class="custom-alert-content">
-            <p id="custom-alert-message"></p>
-            <button id="custom-alert-ok-btn">OK</button>
-        </div>
-    </div>
-
-    <script src="javascript/singForm.js"></script>
+    <script src="javascript/sing.js"></script>
 </body>
 </html>