The inclusion of Arc-KDE fixes T5993

Test Plan:
Arc-KDE theme present in KDE system config (desktop theme, color scheme, window decorator...)
Theme applied with success

Reviewers: Triage Team, sunnyflunk

Reviewed By: Triage Team, sunnyflunk

Subscribers: sunnyflunk

Maniphest Tasks: T5993

Differential Revision: https://dev.solus-project.com/D3577