[Kickoff] Use simpler code for separator color

Summary: The somewhat complex code for separator color in Kickoff creates issues with transparent themes. Separators can become barely visible, defeating the purpose of adding them. Relying on text color on the other hand is a safe bet to ensure visibility.

Test Plan:
Before:

After:

Breeze and Breeze Dark look the same as before:

Reviewers: VDG, ngraham

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D20973