Set dark window borders with dark theme on Linux

[cassidyjames]

Checklist:

• [x] I have checked that there are no issues with similar or the same content

  YOU SHOULD CHECK CLOSED ISSUES ASWELL

• [x] I have checked the FAQ (https://github.com/MarshallOfSound/Google-Play-Music-Desktop-Player-UNOFFICIAL-/wiki/FAQ) and the answer I am looking for is not there
• [x] I have double checked and can reproduce the issue

OS: Linux (elementary OS 5.0 Juno)

GPMDP Version: 4.6.1

Issue Descriptions:

On GTK-based Linux desktops (like GNOME, XFCE, elementary OS, Ubuntu, etc.), apps can request a dark style. When they do so, they get a dark titlebar—for example, Etcher (another Electron app) successfully does this to better match its UI. Some Atom extensions exist to do the same. Google Play Music Desktop Player could similarly set the dark style for its window borders to better blend in on Linux desktops.

Steps to Reproduce:

1. Use a Linux-based OS with a light theme by default (like elementary OS)
2. Enable the dark theme in GPMDP
3. See the mis-matched window borders

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[MarshallOfSound]

Happy to accept a PR that does this if anyone knows how

[cassidyjames]

I'm not super familiar with Electron development, but I might be able to point someone in the right direction; it looks like this was only a one-line change in Etcher: https://github.com/balena-io/etcher/pull/2314/files#diff-c8387f489ada58c8ee2920a9659d31f7R50

[cassidyjames]

So to do it by default, it looks like you'd just update the config at

https://github.com/MarshallOfSound/Google-Play-Music-Desktop-Player-UNOFFICIAL-/blob/23d97992b8d159b6929d546a308f826c85631d33/src/main/configureBrowser.js#L10-L27

But this will obviously require a little more work for doing it dynamically when the dark option is set. darkTheme is a documented option on browserWindow though, which means it should be straightforward for someone who better knows/understands the codebase.