Reuse dialog styling

Adjustments of several styling aspects for the reuse file panel:
* Border style and colours to match the ones used in similar borders and Agora style
* Highlight the trigger also on hover
* Tweak the margins to avoid info on the embed panel to appear next to the edge.

Change-Id: I3a5448e3e81006ae35d2c18a9b2cbc2a6598210e
Merge "Reuse dialog styling"