Decrease skeleton loading opacity

### Requirements - [x] This is a feature request and not a bug report. Otherwise, please create a new [bug report](https://github.com/LemmyNet/lemmy-ui/issues/new?assignees=&labels=bug%2Ctriage&projects=&template=BUG_REPORT.yml) instead. - [x] Please [check](https://github.com/LemmyNet/lemmy-ui/issues) to see if this request (or a similar one) already exists. - [x] It's a single feature. Please don't request multiple features in one issue. ### Describe the feature you'd like The skeleton loader currently has a high contrast with the website background. The idea of this feature is to reduce it's opacity (maybe from 1 to 0.6) to make it visually smoother. Here's a current picture of the skeleton loader on lemmy-ui 1.0.0: <img width="1231" height="871" alt="Image" src="https://github.com/user-attachments/assets/10fa3c01-03ee-4333-bd17-153ec5e6d63c" />