Feature/dark mode (#8026)

* update css stylesheet, replace css properties with css variable

* update css variable

* libraries,favorites,activities and help pages, etc

* wiki dark mode

* sdoc dark mode

* markdown, text editor, etc

* sysadmin pages

* update seafile-ui.css

* update template, css variable

* clean up

* update seafile-ui

* update modal css

* update seafile-ui.css

* delete diff

---------

Co-authored-by: zhouwenxuan <aries@Mac.local>
Co-authored-by: Michael An <1822852997@qq.com>

frontend/src/css/image-file-view.css

@@ -14,7 +14,7 @@
 
 #image-view {
   padding: 1px;
-  background: #fff;
+  background: var(--bs-body-bg);
   border: 1px solid #eee;
   width: auto;
   height: auto;
@@ -32,7 +32,7 @@
   color: #666;
   width: 50px;
   height: 50px;
-  background: #fff;
+  background: var(--bs-body-bg);
   border-radius: 100%;
   line-height: 50px;
   z-index: 1;
@@ -48,7 +48,7 @@
 
 #img-prev:hover,
 #img-next:hover {
-  color: #212529;
+  color: var(--bs-body-color);
 }
 
 .image-zoomer {