Add dark theme support

[gsalinaslopez]

Closes #6. The app will change its color scheme between light (original look) and dark theme based on the OS level settings (Android 10 and higher)

Changes

• Make the main app theme inherit from the DayNight theme as stated in the android developer documentation: https://developer.android.com/develop/ui/views/theming/darktheme#support-dark-theme
• Moved the app theme declaration into the themes.xml resource and added the res/values-night to store the dark theme version declaration. Reference: https://developer.android.com/codelabs/basic-android-kotlin-training-change-app-theme#4
• The about page adds dark theme support by specifying the prefers-color-scheme and color-scheme CSS tags onto the embedded about.html asset. Reference: https://developer.android.com/develop/ui/views/layout/webapps/dark-theme

Screenshots

Main Screen	Settings popup menu	Client Settings	About Page

Notes

The original app's color scheme remains unchanged, now becoming the default "Light Theme"

[gsalinaslopez]

Hi @badaix

Have you had time to take a look at this PR? It should be pretty straightforward and it implements
#6

Thanks!

[badaix]

Hi @gsalinaslopez sorry, I completely missed this PR. I will check ASAP, latest the upcoming weekend :)