fix(FilePicker): Adjust color of placeholder skeletons

[susnux]

Adjust color as suggested by @jancborchardt (I can somehow not request a review from you using GH, but feedback is very welcome 😉 ).

I changed the background to

background: linear-gradient(to right,var(--color-background-darker),var(--color-text-maxcontrast),var(--color-background-darker));
background-size: 600px 100%;
border-radius: var(--border-radius);
animation: gradient 12s ease infinite;

Screenshots

before	after

[jancborchardt]

Looks nice! Is there already a component for this btw or some specific variables? If not it would be good to aligncc @skjnldsv @raimund-schluessler

[susnux]

@jancborchardt I do not think there is a component for this, not sure if it makes sense as it depends on your real content. Maybe just provide the color / background animation of the skeletons as a variable. But maybe someone has an idea how a flexible component can be created for this?