Slack icons are left untouched, theme does not apply

### Specifications
- Hardcode-Tray version : `4.3`
- Distribution : `Kubuntu 22.04`
- Gtk version : `3.24`
- Inkscape version : `` if installed
- Cairo version : `` if installed
- Sni-qt version: `` if installed
- Application name : `Slack`
- Application version : `4.29.149`

### Expected behavior and actual behavior
Running

`sudo -E hardcode-tray --conversion-tool RSVGConvert --size 22 --theme Papirus-Dark --only slack`

should apply the Papirus-Dark icons for slack, but icons are left untouched

### Steps to reproduce the problem
1. Install Slack 4.29.149
2. Run the above command
3. The tray icons does not change

Start by reproducing the reported command with Slack 4.29.149 on Kubuntu 22.04, then trace the Slack-specific handling used by hardcode-tray. The work is complete when the Papirus-Dark theme is applied to Slack's tray icons with the documented command.

Written by the indexing model from the issue text.