So I have dark theme on my kde plasma 5.6 and I want to make my Libre Office look normal, can somebody help me?

i don't know how to make the foreground color of LibreOffice icons light, so i ended up with a light background instead. Tools > Options > LibreOffice > Personalization > Own Theme > Select Theme, search for 'light', select a theme [i prefer 'Mac OSX Light'] and click OK.

There is not much you can do at present but to change GTK3 theme.

Here is the same issue discussion:
https://www.reddit.com/r/kde/comments/4qqpqr/kde_dark_themes_and_libreoffice/

Related issue, with the ruler:
Bug 90214 Summary: Ruler Should be White When Dark Themes Used
https://bugs.documentfoundation.org/show_bug.cgi?id=90214

Bug 48733 Summary: A switch to change LibreOffice UI themes (i.e. dark mode) (see comment 10)
https://bugs.documentfoundation.org/show_bug.cgi?id=48733

Bug 94632 Summary: Light sifr and breeze icon themes for dark OS themes
https://bugs.documentfoundation.org/show_bug.cgi?id=94632

Of course it always good to comment or at least add yourself to CC List so that developers know that more people are interested in it. This may result in giving a bug more priority.