[UI] Enable themed *.png icons again for Linux

[crowning-]

Reference: my first comment here: #761

[UdjinM6]

not tested ACK c755c45

[eduffield222]

I have no linux machines to test this with currently. anyone? :

[crowning-]

Of course I tested it before I created the PR :-)
The change from "true" to "false" in the platform-style just tells the wallet not to overpaint (=color) the icons instead of using the *.png files from the resources folder. The latter was how we did it all the time.

There's no harm merging this PR, and in case somethings looks ugly with the PR our testers will find it faster than we would ;-)