Hmm where to start…, here are my comments and suggestions
Design
The background colors can be further improved. Currently the break in-between the dark elements and light elements is too drastic, making it really ‘glaring’ to the user. An example is the section dividing the Profile UI elements, section “All outgoing email has been…”. A sudden dark divider like that will stick out making it really harsh to read.
The grey and soft colors for call to action buttons such as Reply, likes and the thread navigation widget blends in with the white background, makes it hard for users to locate and use them. Will be really great to increase their color saturation.
Not really into splitting up of the top level menu with logo in the middle. Now currently there are only 7 items in the top menu and it looks lopsided. When there are more navigation items it will be hard to balance. In responsive mode (iPad landscape) the menu drops down, need to extend the width of the navigation. A simple change of color to yellow instead of underline for hovers and actives will make it looks more complete.
iPhone is also the same, maybe a hamburger menu will do nicely to save space.
Thread Navigation Widget
Thread navigation widget on ipad portrait (small desktops) isn’t aligned properly. Doing the margin-left with calc(100vw - 180px) will be better.
Fonts and line-heights
Line heights of body text paragraphs can be increased, it will help in making the text more legible.