fix(cover): Compatible with legacy styles, fixed #677 (#678)

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src/themes/basic/_coverpage.styl

@@ -61,7 +61,7 @@ section.cover
     max-width 500px
     padding 0
 
-  .cover-main > a[data-button]
+  .cover-main > p:last-child a
     border-color var(--theme-color, $color-primary)
     border-radius 2rem
     border-style solid