IMPR UI/UX: Loading spinner pre-styling

[a2nt]

Affected Version

SilverStripe 4 and newer

Description

I think CMS needs some pre-styling to show spinner properly before CSS will be loaded and prevent screen flickering.

That's the way SS CMS looks like before CSS will be loaded.

It can be fixed by adding this snippet directly into html > head

<style>
.ss-loading-screen {
position: absolute;
top: 0;
left: 0;
right: 0;
bottom: 0;
background: #fafbfc;
display: flex;
align-items: center;
justify-content: center;
flex-direction: column;
}
</style>