This adds a new icon for the GTK-based application that adheres (mostly)
to the Gnome Human Interface Guidelines (HIG). The icon is designed to
fit in better with other Gnome applications.
While there isn't a single standard "native" style amongst Linux
applications, I believe this better fits the general Linux desktop
ecosystem over our macOS icon.
The icon itself is undeniably Ghostty. The core design language is the
same and I don't think ayone will mistake it for anything else. I wanted
to keep the brand the same, but making it fit in better aligns with
Ghostty's goal of being "platform native".
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