Theme switch support (#3568)

### What problem does this PR solve?
- [x] New Feature (non-breaking change which adds functionality)

---------

Co-authored-by: Yingfeng <yingfeng.zhang@gmail.com>
Co-authored-by: Jin Hai <haijin.chn@gmail.com>

web/src/pages/add-knowledge/components/knowledge-sidebar/index.less

@@ -11,7 +11,6 @@
       font-size: 16px;
       line-height: 24px;
       font-weight: @fontWeight700;
-      color: @gray2;
       margin-bottom: 6px;
     }
     .knowledgeDescription {