Correct gradients for horizontal scrollbars

[thal]

I noticed that horizontal and vertical scrollbars are both colored with horizontal gradients, which causes the vertical scrollbars to have a raised 3d appearance, while the horizontal scrollbars appear smooth.

This PR changes the horizontal scrollbars to use vertical gradients, so both types of scrollbar appear the same.

Before:

After:

[ochosi]

Crazy that nobody noticed this so far - this issue must be around for ages :) Thanks for looking into a fix, i'll try to review as soon as possible.