Add a grey underline to inactive button

[Svenstar74]

User story

When going to the 'Myths' tab and you get more information about a specific myth by pressing 'Why?', it currently looks like the second image. That should be changed, so that when one button is selected, the other one should also have a grey underline.

Acceptance criteria

• When the 'Flawed Logic' button is selected, there's a grey line below the Sources button and vice versa.