How to get default (light blue) theme

After upgrading to Windows 10, Opus, same as some other applications, got this 'white theme'.
When window panels are partly covering each other, it is often difficult to distinguish the borders.
Also some applications have 'white search boxes'. Anyway, good reasons to adjust colors.
Spent way too much time on this already.

Maybe I am old-fashioned and of course it is a matter of taste of course, but personally, I don't like all these 'dark' themes.
Simple light grey/blue are good enough for me.

Questions:
a) is it okay to just backup my current configuration (Settings->Backup) to save the 'new' layout before applying any new style?
b) how to get the old 'default' theme but keeping my current buttons in place? (meaning: I only want the colors, not to have all buttons replaced)

Thanks

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It's always OK to backup your configuration, but nothing in your Opus configuration will have any effect on system-wide Windows titlebar and control colors.

You want to look at your Windows settings for that, but Microsoft do not let you change much. (I think if you switch into High Contrast mode, they let you change more than you can normally, but it might not look great.) Third party tools like Windows Blinds can change things more drastically, if you don't like what Windows and Microsoft are doing. But this is unrelated to Opus, so this forum is the wrong place to ask about it.

Am talking about Opus, Leo.
I mean the colors/theme like at the bottom (=the old v10 default toolbar).
(As for Windows - there is a tutorial on HowToGeek)

As for Opus: sure, I can pick the same color (hex/rgb) from the default toolbar shown in Opus Help file.
However, when doing so, the 'new' toolbar will then get a dull 'solid' color, whereas you can see
that the default Opus toolbar, there is a little glow/gradient at the top of the toolbar.
Lighter color.

But then again, maybe when restoring the default toolbars will also remove my 'custom' buttons as well?

Anyway, it is a nice to have, but not worthwhile spending a lot of time in case it is too complicated.

The toolbar theme comes from Windows.

You could use a screenshot of a toolbar from the old Windows version to create a background image and use that as the toobar background image.

There's no built-in setting to get the same style without using a background image as it was never part of Opus to begin with.

Thanks and.. sorry for the confusion.
Frankly, I didn't know about all this style stuff being Windows and so on. Never had to worry about it in the past.
So wrongly assumed it was Opus, same as setting toolbar colors so to say.
Indeed other applications also have these all white toolbars.