Remove duplicate CSS and use .bar class in the example

[thled]

I just started using Eww and tried the example. There I noticed small issues and fixed them.

Description

In the example files I removed two CSS class definitions that where duplicated and used the .bar class, which was unused before, on the bar widget.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing