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).


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)