My only comment so far is that the Arduboy buttons offer a different experience in the forums than that in the website.
In here, it's a link to the site (since the logo takes you to the top of the forum), but at the homepage, there's a dropdown with options.
Also, placement is different, specially when using a smartphone (center at the forum, right at the site).
Not a great deal, but would be better if more consistent.
About the gradient: I think it goes with personal taste, so not much to say there. If you like it, it's ok. If not, take it out.
Personally, I'd use stronger colors, but then, I tend to be too bold sometimes.
Oh! Also, since the Arduboy buttons image seems to be outside the forum header, when you activate the editing field, it falls over some of the buttons in the mobile version.
Edit: this is what I'm talking about (over the emoji and settings icons, near my name).