"Bookmark Favicon Changer" can work

drkh

I use Vivaldi 2.0.1309.37 x64 stable in Windows 7.

I would like to customize some bookmark favicons, especially those in the bookmark bar. Searching the Google Web Store turns up Bookmark Favicon Changer 2.21 (BFC). I could not find anything else for the same pupose. Although its latest update is in June 2014, it is still able to do its job in the current Vivaldi stable. However, it needs to be done with the "Shortcut" way and in the "on-line" mode, as follows:

In Bookmarks Manager (CTRL + B), search for the target bookmark and all its duplicates.
Delete all the bookmarks you find, but remember the URL (may be copied to somewhere else). If you have opened the target bookmark to get the URL, make sure you close it afterwards.
From Trash, delete again the same bookmarks to avoid duplicates totally.
Actually I'm not absolutely sure if removal of all the same bookmarks is necessary. But I have tested with this URL: https://codebeautify.org/base64-to-image-converter I made three bookmarks of it, each in different folder. Then I used BFC to choose a custom favicon for one of them, but I failed. No favicon of any of the three bookmarks was changed. Worse still, deleting its shortcut that BFC generates does not help.
Restart Vivaldi.
I don't know if this is necessary. Just in case.
Open the Extension Manager (CTRL + Shift + E).
Go to BFC > Details > Extension options.
Click the Shortcut button (to forsake the default offline way associated with the "Bookmark Favicon" button).

In the first two steps, fill in the URL and optionally the bookmark name.
In step 3, Don't choose the custom favicon you want. Choose a temporary one because later we need to change back to the custom favicon you really want. If at this stage you choose the custom favicon you want, when you choose it later, it will not be used.
Click the Test button in step 4. A new window pops up displaying the web page of the target URL.
Notice the favicon in the tab is not what you chose in last step, but that's all right. Also you may notice the logo icon of BFC appears in the address field. We need it later.
Don't do anything in this window.
Close this window.
In step 5, I choose "First bookmark in 'Bookmarks Bar'." for no particular reason though. I did not try the other positions. If Bookmarks Bar is selected, the shortcuts (also are bookmarks) that BFC generates will be put in the root folder of Bookmorks Manager, perhaps because BFC can not find the bookmark bar of Vivaldi (I guess).
In step 6, click the Add button. The web page of the target URL is automatically opened, with the favicon on the tab STILL NOT being changed to what you want. But we will change it soon.
Drag the tab onto the bookmark bar to make it a bookmark.
In the address field, click the logo icon of BFC to display the "on-line mode" dialogue.
Click the "Change Favicon" button and at this point choose the custom favicon you really want.
After the on-line mode dialogue closes automatically, you can see the favicon in the page tab is changed to the new one, but the one in the bookmark bar is not. That's all right, too.
Close Vivaldi.
You should not open the target bookmark again before closing Vivaldi in order for the new custom favicon not to be replaced by the favicon sent from the web site.
Open Vivaldi again. Now you can see your custom favicon in the bookmark bar. It's no more changed after you visit the web site. Also it sticks after Vivaldi restarts.

If the favicon of a bookmark in the bookmark bar won't change to your custom one, you can copy or move its related shortcut to the bookmark bar, as a replacement.

PS :

You have to keep BFC enabled for it to protect the custom favicons.
For some reason BFC can not change the favicon of Youtube, even when Javascript is disabled for it from its Site Settings. But in BFC's custom favicon database Youtube is there with the custom favicon for it. I exported the custom favicon database (click the "More" button) and then imported it, replacing Youtube's custom icon with the same one. After that, the official Youtube favicon in the bookmark bar did change to the custom one. But after Vivaldi restarted, the official favicon was back. Subsequently no matter how many times I did importing, Youtube's custom favicon no longer showed. Anybody has any idea?
For a changed favicon all the buttons in the on-line mode dialogue do no work.
Cannot use BFC for bookmarklets.
I have heard that Chromium-based browser must use 16x16 PNG's for favicons. So that I did. But the author says the following:

You can use any image file or any image URL that browser support as a favicon.
Support every size of image file. There is no different in disk space requirement between big and small image size. Every image will be resized and changed to 16x16 pixels png files and saved.
After changing favicon, there is no need for that favicon file to exist again.

Mod Edit: Fixed Formatting. See Markdown Help

Thanks for the mod edit.