Outline Y-Sweet logo (#346)

The Y-Sweet logo has a white outline as part of the logo with some
gradients that add depth. On white, the outline is completely invisible
except for the gradients, which look like flaws. This adds an outline to
the logo so that the outline is apparent on a light background. The
outline matches the default GitHub "dark mode" color, so it is
completely invisible on the default dark mode.

Before:

<img width="916" alt="image"
src="https://github.com/user-attachments/assets/6cd5c1f9-6cd4-4e93-8722-21c3b265e90c">

After:

<img width="922" alt="image"
src="https://github.com/user-attachments/assets/abc48e0b-5e74-46f5-8edd-5db0bb73ebcb">