Table: use $--table-header-background to define header background(#8871)

$--table-header-background is defined but never used

packages/theme-chalk/src/common/var.scss

@@ -480,7 +480,7 @@ $--table-text-color: $--color-text-regular !default;
 $--table-header-color: $--color-text-secondary !default;
 $--table-row-hover-background: $--background-color-base !default;
 $--table-current-row-background: $--color-primary-light-9 !default;
-$--table-header-background: $--color-text-secondary !default;
+$--table-header-background: $--color-white !default;
 $--table-footer-background: $--color-text-placeholder !default;
 $--table-fixed-box-shadow: 0 0 10px rgba(0, 0, 0, .12) !default;
 

packages/theme-chalk/src/table.scss

@@ -168,6 +168,7 @@
     overflow: hidden;
     user-select: none;
     text-align: left;
+    background-color: $--table-header-background;
 
     div {
       display: inline-block;