chore: refactor css code style

assets/css/_page/_single.scss

@@ -270,20 +270,6 @@
     @import "../_partial/_single/code";
     @import "../_partial/_single/admonition";
 
-    .mermaid {
-      width: 100%;
-      margin: 3% auto;
-      text-align: center;
-
-      @import "../_mermaid/neutral/index";
-
-      .dark-theme & {
-        @import "../_mermaid/dark/index";
-      }
-    }
-
-    @import "../_aplayer/dark.scss";
-
     .echarts {
       width: 100%;
       height: 30rem;

assets/css/lib/VERSION

@@ -4,3 +4,4 @@ gittalk@1.5.0
 katex@0.11.1
 aplayer@1.10.1
 animate.css@3.7.2
+mermaid@8.4.2

assets/css/mermaid.scss

@@ -0,0 +1,11 @@
+.mermaid {
+  width: 100%;
+  margin: 3% auto;
+  text-align: center;
+
+  @import "lib/mermaid/neutral/index";
+
+  .dark-theme & {
+    @import "lib/mermaid/dark/index";
+  }
+}