docs: update theme preview image and fix some docs error (#151)

assets/css/_core/_base.scss

@@ -58,10 +58,6 @@ body {
   }
 }
 
-b, strong {
-  font-weight: bold;
-}
-
 a {
   color: $global-link-color;
   text-decoration: none;
@@ -87,42 +83,4 @@ b, strong {
   }
 }
 
-#dynamic-to-top {
-  display: none;
-  overflow: hidden;
-  width: auto;
-  z-index: 100;
-  position: fixed;
-  bottom: 2rem;
-  right: 2rem;
-  top: auto;
-  left: auto;
-  font-size: 1.4rem;
-  line-height: 1.8rem;
-  padding: .6rem .6rem;
-  color: $global-font-secondary-color;
-  background: $header-background-color;
-  border: 1px solid $global-border-color;
-  border-radius: 2rem;
-
-  &:hover, &:active {
-    color: $global-font-color;
-    background: $global-background-color;
-    cursor: pointer;
-  }
-
-  &:active, &:focus, &:hover {
-    outline: none;
-  }
-
-  .dark-theme & {
-    color: $global-font-secondary-color-dark;
-    background: $header-background-color-dark;
-    border-color: $global-border-color-dark;
-
-    &:hover, &:active {
-      color: $global-font-color-dark;
-      background: $global-background-color-dark;
-    }
-  }
-}
+@import "../_partial/dynamic-to-top";