increase opacity of buttons, fix for dark backgrounds like header in pub...

...lic share, fix #1188

Please review @DeepDiver1975 @Raydiation 

This was actually a bit of a pain because all button styles needed to be changed. Reason being their background colors have a slight transparency to fit better on varying light backgrounds. Only problem is it didn’t work at all on dark ones, of which the public share download button in the header seems to be the only occurence.