Icon design

[brunofin]

According to Flathub guidelines, the icon is not passing the following checks:

• [ ] Has primary brand colors (https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#has-primary-brand-colors and https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#good-brand-colors)
• [ ] In line with contemporary styles (https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines/#in-line-with-contemporary-styles)

Icons are needed for those in the data/icons/hicolor folder.