Fix dark theme table header text color

Author: GuillaumeGomez

templates/style/_themes.scss

@@ -62,7 +62,7 @@ html[data-docs-rs-theme="dark"] {
   --color-warn: #e57300;
   --color-background-input: #f0f0f0;
   --color-table-header-background: #545252;
-  --color-table-header: #000;
+  --color-table-header: #c0c0c0;
   --color-search-focus: #078dd8;
   --chart-title-color: #c0c0c0;
   --chart-grid: #4e4e4e;