The best alternatives to Dimensions Toolkit are Pastel, W3C Mobile Checker, and OneClickTest. If these 3 options don't work for you, we've listed a few more alternatives below.
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Dark Mode Design is an inspiration resource showcasing beautifully designed websites that are either exclusively in dark mode or have the ability to switch.
Validatorapp is a platform for creating custom demo checkout and subscription pop-ups for your website. It is designed for validating your new ideas or MVPs before fully building or releasing them. It can be added to your website with just a few minutes.
Without the presence of devices, it allows you to check the display of the site layout at the same time on several screens, as well as compare the layout with design layouts. We will tell you more about the capabilities of the service in the comments.