You won't be able to modify the Craft CMS core CMS files (especially if you are using composer as the entire CMS is deployed in the vendor folder), but you can load in your own resources like CSS and JS through an assetbundle in a module, or you can use a plugin like: https://plugins.craftcms.com/cp-css to load custom CSS.

Although, if you think there's an issue in core Craft CMS, you can report an issue to the GitHub page for comment from the developers: https://github.com/craftcms/cms/issues.

The CSS rule you have identified will be related to accessibility for screen reader users to skip to the main content on the page.

You won't be able to modify the Craft CMS core CMS files (especially if you are using composer as the entire CMS is deployed in the vendor folder), but you can load in your own resources like CSS and JS through an assetbundle in a module, or you can use a plugin like: https://plugins.craftcms.com/cp-css to load custom CSS.

Although, if you think there's an issue in core Craft CMS, you can report an issue to the GitHub page for comment from the developers: https://github.com/craftcms/cms/issues.

The CSS rule you have identified will be related to accessibility for screen reader users to skip to the main content on the page.

Edit: This may have been a bug with Craft CMS, see: https://github.com/craftcms/cms/issues/11375