@Xen - I did have a problem with the colors, but I wasn't using a high contrast theme at the time. Changing to use interface style system default was what I found to work as well.
I am using a high contrast theme now, but I use the high contrast white because I found a lot of apps and web pages do not work when a high contrast black theme is used. I had to change almost every color to get rid of the white and Calibre does work well with it. A lot of web pages still have things missing, but I turn high contrast off and on to see where I need to click and then memorize for the next time. MobileRead is one of the sites that works fine with my current high contrast theme. I have the background color set to a medium lilac color so if your problem with white is not severe maybe you could try changing the windows background to something not white, but not too dark.
For the toolbar in Calibre you can change the position of the items you use most and put them where you don't need to get to the second row. There's also a plugin called Favourites Menu you can add to the toolbar to put the things you use, but don't use too often, so they are easier to get to. Not exactly favorites, but it does group things so you can get to the easily.
Here's a screen print of Calibre using my high contrast theme, and also a screen print of MobileRead advanced reply screen - you can see all the editing tools including the attachments icon:
