Design review of file sorting

[jbtbnl]

A couple of recommendations for the recently merged file sorting #8041:

• bring back the grey-out-while-loading animation instead of the instant blank page
• remove on-hover background color of header columns
• bring back the #999 default color for column headers (arrow icon as well)
• column headers solid black on hover (arrow icon as well)

CC @owncloud/designers @PVince81

[PVince81]

Feel free to take care of this yourself, I'm busy with the sidebar and other tasks at the moment.
Thanks.

[jancborchardt]

This is a good Junior Job for you to get started as well @ruchitarathi @Apophenia, cc @cosenal

[jancborchardt]

All fixed in a recent PR of mine. :)