New feature: Theme handling suggestion

[Sp3EdeR]

Hello devs,

Caprine currently supports custom themes, however current use-cases require one to copy-paste the theme CSS into the local file fully. When Facebook updates Messenger's code, it is possible that some themes may break. For example the Discord theming community already works around this issue by pushing themes to Github pages, and importing them into the client using the following CSS syntax:

@import url("https://sp3eder.github.io/black-messenger-for-caprine/theme.css");

I tried to use the above in Caprine, however it appears not to work using the current CSS injection mode. According to the debugger, no request is sent for this file by the client. I think it would be a useful feature to add support for this syntax, to support a more stable theming system.

[mquevill]

When Facebook updates Messenger's code, it is possible that some themes may break.

^ Not only themes, but some of Caprine's core functionality breaks when there's back-end changes to Messenger...

I haven't had much luck into figuring out why this is not possible with webContents.insertCSS() function that Caprine uses to add the CSS, but could be worth looking into.