I was hoping to see it a little more material design-esque, but this is alright. I'll give a few opinions.
I do agree that it's a little too bright, maybe make the shadows a little more prominent on the outside.
The table cell separation bothered me pretty bad so I had to set that to 0px right after I installed it.
The banner has that weird black bar on the bottom, maybe re-create it or just crop that part out.
I find it hard to see the difference between different posts, even when the table cells were separated.
The stats on the right side of the main forum navigation ("Hey, TheBlackParrot, you have ### messages") are a bit off. The top line is a couple of pixels too far to the left.
The quote buttons on posts stand out too much, they draw attention away from the post content.
Some parts of the forum aren't persistent width-wise (e.g. the main post wrapper and the headers/footers/etc.)
Maybe bold or increase the size of some main text elements? (breadcrumbs, board titles)
I can possibly mess around with the theme a bit and send you some stuff I changed if you want. I like the theme, everything just blends in way too much.
EDIT: you know rgba([0-255],[0-255],[0-255],[0-1]); is a thing right