Forget the decoration, neither GitKraken nor PHPstorm are necessary.
To get started, copy one of the themes:
cd themes
cp -rp community-theme-default my-new-theme
Which one doesn't matter. Both are technically almost the same, just (S)CSS is quite distinct.
Then edit config.xml inside the new theme with your favorite code editor (vi, nano, geany, gedit, sublime, whatever):
cd my-new-theme
geany config.xml
Edit fields name and directory in the upper few lines. Each appears twice. Name is arbitrary, directory should match the actual directory. Save the file. Why these edits are necessary? Well, PrestaShop decided it this way 10 years ago and changing this would easily break existing themes.
At this point you can install the theme already. It'll look like the one you copied, but it's the new one.
Then you can build the theme:
npm install
npm run compile-css
npm run format-js
npm run copy-index
npm run clean-up
First command is needed only once, second command is the essential one, the other ones are tidying up code a bit.
As you didn't change anything yet, a new build shouldn't change anything.
Now you're ready for editing the theme. Template file (.tpl) file edits take effect immediately. JavaScript files as well. Just reload the page in your browser and the change is there.
Regaring CSS: don't edit .css files, but the .scss ones. After each edit, rebuild the theme, then reload the page in the browser.
At this point you're in the middle of development already. Cheers!