Fix compatibility of build_theme.sh with Dash

Summary:
The script build_sh didn't work with Dash necause of "[[". When running it, I got the following:

$ ./build_theme.sh
Installing into /home/gepardo/.local/share/themes/Breeze
./build_theme.sh: 111: [[: not found
cp: cannot create directory '': No such file or directory

Test Plan: After changing "[[" to "[", it worked fine and installed the theme

Reviewers: cblack, svuorela

Reviewed By: cblack, svuorela

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D24447