The stx extension contributed the setting css.remToPxRatio and the command css.reload. css. is the namespace of VS Code's built-in CSS language features (css.validate, css.lint.*, css.hover.*, ...), so a setting there can collide with one VS Code adds, and it shows up under VS Code's CSS section in the Settings UI rather than under stx.
Fix: rename them into the extension's namespace, next to the other utility-class settings:
css.remToPxRatio->stx.utilityClasses.remToPxRatiocss.reload->stx.reloadUtilityClasses("stx: Reload Utility Class Configuration")
A value set under css.remToPxRatio keeps working: it is read when the new key is not set anywhere, silently, since there is nothing the user has to do. The Stacks extension, which declares the same contributions from @stacksjs/stx-vscode's contributes.json, picks the rename up with bun run sync:stx.