Implement a dark tray icon for light themes

Feature Suggestions
,
#21

Thank you.
I’ve opened a PR here https://github.com/Foundry376/Mailspring/pull/2395
I think it would especially benefit from someone else testing it, as I don’t have a testing setup for all the Linux desktop environments

#22

For now, I am replacing the white icons with inverted ones following instructions from Reddit user void_matrix found here: https://www.reddit.com/r/linux4noobs/comments/1f9zkdz/how_to_change_the_mailspring_notification_icon_in/
In may case, it was in /usr/share/mailspring/resources/app.asar/internal_packages/system-tray/assets/linux/

I made a script to automate the whole thing, requires npm and imagemagick to be installed

#!/bin/bash
if type npm convert &> /dev/null; then
sudo npm install -g asar
sudo cp /usr/share/mailspring/resources/app.asar /tmp/
sudo cp -R /usr/share/mailspring/resources/app.asar.unpacked /tmp/
sudo chmod 777 /tmp/app.asar
sudo chmod -R 777 /tmp/app.asar.unpacked
asar extract /tmp/app.asar /tmp/mailspring
convert -negate /tmp/mailspring/internal_packages/system-tray/assets/linux/MenuItem-Inbox-Full.png /tmp/mailspring/internal_packages/system-tray/assets/linux/MenuItem-Inbox-Full.png
convert -negate /tmp/mailspring/internal_packages/system-tray/assets/linux/MenuItem-Inbox-Zero.png /tmp/mailspring/internal_packages/system-tray/assets/linux/MenuItem-Inbox-Zero.png
sudo rm /tmp/app.asar
asar pack /tmp/mailspring /tmp/app.asar
sudo cp /tmp/app.asar /usr/share/mailspring/resources/
else
  echo "This script requires the following packages to run: npm imagemagick"
  type npm &> /dev/null || packages+="npm "
  type convert &> /dev/null || packages+="imagemagick "
  echo "Missing: $packages"
fi