On Windows the file is located at:
```bash
C:\Users\username\AppData\Local\Programs\Microsoft VS Code\resources\app\out\vs\workbench\workbench.desktop.main.css
```
Currently (Version 1.90.0), the setting to change the color of the 'M' for git-modified files is:
```css
.monaco-decoration-itemBadge-kkyaq5::after{color:#9ece6a !important}
```
If it changes in the future, open the VS Code Developer Tools (⌘ ⇧ P) and inspect the element to see the correct class.