chore(docs): add site theme for dark mode (#84)

Author: ota-meshi

docs/.vitepress/theme/style.css

@@ -1,9 +1,25 @@
 :root {
-    --vp-c-brand: #4b32c3;
-    --vp-c-brand-light: #8080f2;
-    --vp-c-brand-lighter: #a0a0f5;
-    --vp-c-brand-dark: #341bab;
-    --vp-c-brand-darker: #341bab;
+    --color-primary-200: #dedeff;
+    --color-primary-300: #ccccfa;
+    --color-primary-400: #b7b7ff;
+    --color-primary-500: #a0a0f5;
+    --color-primary-600: #8080f2;
+    --color-primary-700: #6358d4;
+    --color-primary-800: #4b32c3;
+    --color-primary-900: #341bab;
+    --vp-c-brand: var(--color-primary-800);
+    --vp-c-brand-light: var(--color-primary-600);
+    --vp-c-brand-lighter: var(--color-primary-500);
+    --vp-c-brand-dark: var(--color-primary-900);
+    --vp-c-brand-darker: var(--color-primary-900);
+}
+
+.dark {
+    --vp-c-brand: var(--color-primary-400);
+    --vp-c-brand-light: var(--color-primary-300);
+    --vp-c-brand-lighter: var(--color-primary-200);
+    --vp-c-brand-dark: var(--color-primary-700);
+    --vp-c-brand-darker: var(--color-primary-800);
 }
 
 a > img {