Fix blurry icons in KColorSchemeEditor

Summary:
When using fractional scaling, bottom icons are blurred. This patch fixes the issue.

BUG: 418165

Test Plan:
No side effects.

Before:
{F8289358}
After:
{F8289369}

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29481