When we install a theme other than base theme like Threads, Horizon etc.. Then there will be an pre contain extension will be coming along with the theme installed using the bundle mostly with the same name of the theme like Threads extension, Horizon extension. This extensions can not be fetched and able to edit So because of this we cannot modify the css of the extension and thats the reason our other extensions sass and theme sass will colloid with the css property of this extension. So It would be better to de-activate the extension and work with other extensions otherwise it will cause overlapping issue in the website. I have faced issues because of this extension screenshots have added below.
Here the UI issue is caused in the checkout page is due to the extension. This is dektop view here the tab view and desktop view is overlapping with each other.