Cosmetic fixes for build_theme.sh

Summary:
I saw the changes introduced by https://phabricator.kde.org/D18287. It's a really good idea :)

The commit contains some minor fixes into the build script (like quoting "$4" parameter)

Test Plan: Builds on my machine. The changes are minor and should not affect anything.

Reviewers: davidedmundson

Reviewed By: davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18314