Yeah, that's definitely not the way to approach this. It looks like you are fairly young and new to this game, so let me give you some advice around constructive criticism, good open source contribution, and making good technical decisions.
First: As Laurie pointed out, comments like "yuck" don't further any discussion. They do the exact opposite. Instead of making people want to work with you, it makes people shut down and not take any of your comments to heart. Instead, try leading with some positive comments - "this is really cool" or "I really like this idea!". Then make sure your feedback is what the person is actually asking for. In this case, Ashlee was asking for feedback on what things people would like to see. You instead took it upon yourself to make those contributions. Ashlee posted her work on Github and you definitely should be empowered to suggest changes, but you need to do so in a constructive way.
That brings us to the next point. Contributing to an open source project is a privileged granted to you by the maintainer of that project. Now, Ashlee hasn't placed a license on the repo and you are within your rights to fork said repo. But a certain amount of respect should be had for the owner. Open source work is like a garden - we all have to tend to it well to make sure it continues growing. If we all try to plant our own plants wherever we want, nothing has good growth potential. A good option here might have been to make a constructive, positive issue on the repo, outlining some possible features you would like to see. Then a comment along the lines of "I'd love to help with this! I'll start on a PR to add X feature" would be a great option to get involved.
This brings me to my next point - not all the features you think are the right ones are actually the right ones. This app doesn't need CSS grid. The layout collapses on mobile, but that could be fixed with about 2 lines of code. You aren't "curing" anything with your code - you aren't a doctor and your code isn't the magical cure to the woes of Ashlee's project. The project loads at most sizes and there is a good argument to be made that it has no reason to be loaded on a mobile device. That's definitely not a feature I would prioritize off the bat.
Your other ideas are good - palettes would be nice and a way to get the CSS variables is great, but your presentation leaves a lot to be desired. "Making the code nicer" isn't a feature, it's a personal preference. And the app doesn't really need SEO - it might be nice, but the Github page will do a good amount of that work. Using CSS borders might be nice, but isn't 100% needed - hr's are fine.
I don't want to drive you away from contributing to this - you have good thoughts and I'm sure you are enthusiastic about working on this project, but your presentation could use some work. If I were you, I'd make an apology for coming off offensive and then rework your comment to be constructive. This is a great learning opportunity moving forward! You have a good way to go in your career and this is a moment you should look back on as growth as an individual.
You definitely have a good point. I'm certainly not the most positive person. And when it comes to explaining/commenting on something, I'm actually quite awful. I often have controversies IRL, so I definitely need to work not only on my presentation, but also on my attitude.
It's ok - it can be hard to do. Positivity is a skill as is constructive criticism! You can work on both of those things! If you are less comfortable doing it IRL, online is a great place to learn and grow. One other piece of advice I have is to approach everything from the standpoint of trying to learn. If your explanation is along the lines of "Here is what I am thinking on this, what do you think?" you are leaving room in the conversation for the other person. If you can do that, you'll be golden. Feel free to reach out to me anytime if you are looking for feedback. I'd be happy to help you out!
Also, no one's code is perfect. For example, I can't say that my code is perfect. Everyone's code has some flaws, like everything in the universe.
Right - that means that you need to approach everything from a standpoint of thinking about people. Code is way more people-centric than it is machine-centric. There isn't ever a "right" answer, so approaching it from trying to learn, not trying to be correct.
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