Theme improvements

[AnouckColson]

Where on docs.platform.sh should be changed?

https://docs.platform.sh https://docs.upsun.com

What exactly should be updated?

Theme flaws that need fixing:

• When clicking an index page (such as Reference > Add services), the menu from the side nav doesn't unfold as it used to.
• When clicking a third-level item in the side nav (such as Manage environments > Back up an environment), upon loading the target page the menu from the side nav folds up again, so you can't see which page you're on in the section anymore.

See links in context for videos from reporters.

Additional context

No response