It is not possible to customize the cPanel default login theme and I do not think Hosting24 can change this because cPanel is not developed by them.
As reseller you can use the branding editor to customize the screens your clients see. It allows you to edit the header and footer images in each theme, as well as the main page icons for the style. To do this, create a new branding style, then edit it by adding your custom images. You can also make an archive of all your images, and upload it.
YouHosting has developed all software used themselves, including the Control Panel. So, YouHosting is very different from Hosting24.