Left before, right after:
<img width="1728" height="370" alt="image"
src="https://github.com/user-attachments/assets/3229ba26-18e6-415e-8df3-25fa5fea43aa"
/>

Closes https://github.com/zed-industries/zed/issues/61976

https://github.com/zed-industries/zed/pull/61134 picked a wrong color
for `variable.parameter` that exactly matched `variable.special` and
caused visual conflicts.
Instead, use `variable` for such colors.


Release Notes:

- Fixed Gruvbox parameter colors