Add Themes to Wallet

[CaveSpectre11]

Dash has several themes that was added after the divergence of Pivx; which could be very nice to provide different backgrounds and/or themes to the wallet. Thinking "Light (default); what will be updated in v1.2)", "Dark", and possibly 3 background based themes.

Changes to dash seem contained in two PRs:
dashpay/dash#600
dashpay/dash#612

How it is controlled can be seen in Dash Display Options:

https://docs.dash.org/en/stable/wallets/dashcore/interface.html

This will probably require the ability to have a transparent block under the active areas of the wallet, which mutes the background a bit and allows the text to be better read.