Update markdown code display

[mfg92]

At the moment code is rendered like this (Windows, Firefox 61):

I think always showing the scrollbar doesn't look good. How about hiding it if it is not needed (first commit / d86ba2f):

Furthermore, I would also add a darker background to the code and make its width 100% of the content area (second commit / 706992a):

I have no background in design etc. so I am interested in your opinion @vickylai on these changes.

Test markdown:

Bla bla...:
```
test.code();
```

..bla bla.

[mfg92]

Now I noticed that code is rendered completely different if I specify a language for pygemnts in markdown.
So this is the main problem.
When pygemnts are used and the code is wrapped in a 'dive class="highlight"', always showing the scrollbar looks OK to me. But for my taste the width should be fixed at 100%.

[victoriadrake]

These changes are most definitely better!

Yes, the theme's default styles for pre will be overridden if these two lines are present in config.toml:

pygmentsStyle = "monokai" # Any theme name from: https://help.farbox.com/pygments.html
pygmentsCodefences = true

I agree about the 100% width.

[mfg92]

Thank you for your fast response.

I noticed a minor flaw in your changes in saas/_elements.sass:
Now there are two entries for .highlight. I think the lower one is not needed anymore.

[victoriadrake]

You're right. Thank you!