After 1.8 Update Browser Theme is blue only.
Top bar is not changing color as it was used to do. Some websites after the 1.8 update shows blue color only as theme, (www.sahibinden.com was yellow but its now blue, www.flypgs.com was orange and its blue too. )
I can confirm that for the first page (sahibinden) the accent color stays default. The second one (flypgs) turns bright-orange in the latest Stable but for some reason it remains default in the latest Snapshot. The second one should work, as it defines its theme color in the header:
<meta name="theme-color" content="#fdb712">
The first one lacks that property,
hence the theme stays default. I think that earlier versions of Vivaldi used to pick the accent color basing on the site's logo predominant color, whereas the latest ones use only the meta tag
Actually, it seems to be quite random. I just tried creating a test page with
theme-colordefined as bright-green but for some reason, it picked bright-blue from the favicon (which contains equal amounts of 3 different colors and a little addition of the fourth color). After reopening the page (all tests performed in private windows) it picked the accent color correctly (from the meta tag).
Tested in Vivaldi 1.8.770.50 (Stable) / 1.8.770.46 (Snapshot) (both 32-bit on Windows 10 x64).
Sahabinden has no meta for theme-color and a broken favicon, so the tab color is the default of the theme.