There exist CSS classes current-cat and current-cat-parent allowing different styling of Current Category in the "Category" widget.
There exist CSS classes current_page_item, current_page_parent and current_page_ancestor allowing different styling of Current Page in the "Pages" widget.
But there are no CSS classes supporting Current Post / Archive / Tag in the "Recent Posts" / "Archives" / "Tag Cloud" widgets respectively.
I suggest:
- adding the new CSS class current-post allowing different styling of Current Post in the "Recent Posts" widget;
- adding the new CSS class current-archive allowing different styling of Current Archive in the "Archives" widget;
- and adding the new CSS class current-tag allowing different styling of Current Tag in the "Tag Cloud" widget.