Till now Firefox browser has the best (gray) theme for Windows 10 instead of showing plain white title bar, Opera desktop team is testing a new theme for Windows 10 with developer 33 version.
You can’t find any new changes in theme when compared with the current one, except the scrollbar, to see this change, you need to be using Windows 10 RTM version or 10525 build.
Enabling new Alternative native theme for Opera on Windows 10
2. Select ‘Enabled’ from the drop down and restart the browser.
DNA-42467 Add NativeThemeAlt for Win10 (which falls back to NativeThemeAura)
Here is a warning for you: don’t turn on #autoupdate-notifications flag as it is causing the browser to crash for every 10 minutes, opera devs are working to fix this.
The new Opera developer 33.0.1967.0 released today comes with fixes to crashes (except the one mentioned above) and bugs.