add: 背景色をtransitionさせる

src/app.css

@@ -1,3 +1,12 @@
+@property --theme-color {
+    syntax: "<color>";
+    inherits: true;
+}
+@property --back-color {
+    syntax: "<color>";
+    inherits: true;
+}
+
 /* theme color */
 body, body.light {
     --color-scheme: light;

src/routes/(DefaultStyle)/+layout.svelte

@@ -51,6 +51,7 @@
         display: flex;
         flex-direction: column;
         background-color: var(--back-color);
+        transition: background-color 1s;
         min-height: 100vh;
     }
 
@@ -76,7 +77,7 @@
         & header {
             background-color: var(--theme-color);
             height: 100%;
-            transition: height .4s, transform .4s;
+            transition: height .4s, transform .4s background-color 1s;
             &.hide {
                 height: 40px;
                 @media(width<1000px) {