@ovan
We're sorry for the experience you had trying out Vivaldi.
We try to pick good defaults, but since needs and tastes vary, we provide (a lot of) settings for you to customize the browser according to your preferences. Admittedly, it may be a little overwhelming at the beginning, but once you've set things up, the browser will be perfectly tailored to you.
What would you like to see when opening a new tab instead of the Start Page?
I also tested pasting hex values to Theme settings and it worked as expected. Could you describe the issue you encountered?