More UI polish for Dark Mode
More UI polish for Dark Mode

* Tweak contrast of background linear gradient
* Tweak background colors slightly darker
* Tweak foreground colors slightly lighter
* Tweak buy/sell buttons color slightly darker
* Tweak chart axis line and label font colors

bitsquare/bitsquare,bisq-network/exchange,bisq-network/exchange,bitsquare/bitsquare