Different skin / theme for blockly

This is a shallow point, but it might be good to replace the default blockly theme with something that looks different.

Here's why: students will immediately recognize the blockly look and feel, and associate it with "Hour of Code" activities they've done using https://code.org/learn We don't really want them to compare this to Hour of Code activities. Hour of Code activities are heavily branded with commercial partnerships involving favorite video games and TV shows, and are designed to require only a minimum effort to get a slightly customized (but mostly pre-made) result. The goal should be for this to feel like something different, where the kids are creating something entirely their own. But when the UI looks the same, it will make many kids think it's just the same tool.

Not a high priority, though.

Im Issue ist keine Datei, kein Test und kein Einstiegspunkt angegeben. Beginne damit, die Blockly UI und ihre aktuelle Theme-Integration zu finden, und ermittle dann, wie Themes konfiguriert werden. Als erledigt gilt die Aufgabe, wenn die Oberfläche ein eigenständiges visuelles Theme statt des standardmäßigen Blockly-Erscheinungsbilds hat.

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