Hi everybody,

After a lot of digging, I have found a solution that works for me:

1.- Go to opera://flags/ and turn on 'Force Dark Mode for Web Contents' to Enabled

This fixes most of the issue (e.g. about:blank becomes dark and most of the flashbangs disappear). In my case, there is still some brief white screen (of shorter duration than the default and much less annoying).

Enjoy!

PS: If someone wants to keep digging for a complete 100% solution, this might be a good path to go down to and experiment with:

2.- Installing the Stylus extension and following the steps of this stackexchange answer, which in short are:
a.- Click on the styles icon > Manage > 'Write new style'
b.- paste something like this (won't solve the issue, just an example):

@-moz-document url-prefix(about:blank) {*{background-color:#000000;}}

c.- click on 'overwrite style'
d.- give it a name on the upper left > save
e.- restart the browser just in case

Please report back if you do find the ultimate solution, otherwise I'll be switching over to FF for my late night browsing (which is a pity cause it loads sites slower than opera)