Accessibility issue with section pulldown menu in courses

It's not possible to identify the pull-down menu.

Steps to reproduce

1. Create a course and set the "Course layout" to "Show one section per page".
2. Access to any of the topics (Topic 1).
3. Use the keyboard to navigate and try to change the Topic using the "Jump to..." selector.

Expected behaviour

You can select the topic using the keyboard.

Current behaviour

Nothing happens when a topic is selected from the "Jump to..." list.