Awesome! Thanks for sharing it here. I don't have much to add, your 404 page is missing css.

Curious where you got the idea to do try/except: return 500.html? I'll sometimes return str(e) when testing and developing, but that's definitely not the ideal in production.

Instead of that, I would recommend you have the following, much like your 404 handling:

@app.errorhandler(500)
def error_500(e):
    return render_template('your500template.html')

Also, I would not suggest you get in the habit of returning str(e) from the error on a production site. I do it while debugging, and often forget to stop doing it. For hackers, being able to see the errors is a major help to figuring out what to do next. Instead, you may want to start up a log file that contains things like 404s, and 500s that people come into contact with.

Finally,

@app.route('/projects')

means the user can only go to literally /projects. If they do /projects/, it will result in a 404. For that reason I tend to end with a forward slash. Chances are, it wont be a big deal most of the time, but sometimes I am manually typing a url and include the ending slash. If that gave me a 404, before programming, I would have not tried without the slash. If you make your urls:

@app.route('/projects/')

...someone will be successful regardless of whether or not the end with the slash.

Your design is really nice, much better than anything I can come up with! I would also suggest you include the actual website itself as one of your "projects."