Use symbolic icon in intro page

Summary:
Sorry for the large amount of smaller changes but on dark themes the right ">" is too dark to be noticeable - just look at the screenshots:

old:

new:

on light theme:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24097