feat: add filters to the cover image (#89)

#### What type of PR is this?
/kind feature

#### What this PR does / why we need it:
为封面图增加一个暗色调滤镜，解决在亮色调背景图的情况下标题无法看清的问题。

#### Which issue(s) this PR fixes:
Fixes #86 

#### Does this PR introduce a user-facing change?

```release-note
为封面图增加暗色调滤镜
```

templates/modules/footer.html

@@ -7,8 +7,10 @@
         class="mr-3 h-8 w-8"
         th:alt="${site.title}"
       />
-      <span class="self-center whitespace-nowrap text-2xl font-semibold dark:text-slate-50" 
-            th:text="${theme.config.footer.title ?: site.title}">
+      <span
+        class="self-center whitespace-nowrap text-2xl font-semibold dark:text-slate-50"
+        th:text="${theme.config.footer.title ?: site.title}"
+      >
       </span>
     </a>
     <th:block th:if="${not #strings.isEmpty(theme.config.footer.menu)}">