In the popup panel that appears when clicking Add for effects, the sub-section headings such as "Sharpen" and "Color Adjustments" are very dark on the dark background.
They are hard to read at any monitor brightness level.
I don't know if it's just me, but those look dimmer than usual. If you'd like us to take a closer look at this, do drop an email to support@pixelmator.com. Either way, I think those categories are probably something we could take a look at and find a better solution for!