Updated the style

[boldtrn]

Fixes #60

I updated the style a bit. I personally like it better that way :). But these changes are always quite opinionated.

That's how the style looks right now:

What I changed:

• I added a background transparency
• I removed the stroke for the top border, as this can look a bit ugly for long routes (~ >300km), depending on the width of the graph
• The axis and labels are now black to make it easier to read
• I fixed the font-size for mouse hover box. Before the font-size would fallback to the default font size of the document and there was no easy way to fix it.

WDYT @TimMcCauley should we merge this? Especially the black axis and background transparency are a bit opinionated. I could remove these changes from the PR.

[TimMcCauley]

@boldtrn I agree with these changes, the opacity makes it easier to read and removing the stroke is absolutely fine! Thank you.