Fix unreadable Falkon logo text when using a dark theme

Summary:
BUG: 400262
https://bugs.kde.org/show_bug.cgi?id=400262

This puts a light rounded rectangle behind the logo, so the logo text is readable when using a dark theme.

Test Plan: Open the about window (Application Menu -> About Falkon).

Reviewers: Falkon, drosca

Reviewed By: Falkon, drosca

Subscribers: drosca, falkon, Falkon

Tags: Falkon

Differential Revision: https://phabricator.kde.org/D17307