Close #25051 [referenced answer](https://stackoverflow.com/questions/10813581/can-i-replace-the-expand-icon-of-the-details-element/69722686#69722686) for marker overwrite. One limitation is that fomantic does not have hover and active effects for the vertical submenu ([reference](https://fomantic-ui.com/collections/menu.html#sub-menu)). And we might need to overwrite some styles if hover and active effects are needed. Update: Used `data:image/svg` instead of `marker` content. And adjusted styles for hover effect. Take admin settings as an example: https://github.com/go-gitea/gitea/assets/17645053/63f69823-ef43-47d5-a518-544b5ea35ba6 --------- Co-authored-by: silverwind <me@silverwind.io>
