At home, I'm running Geany 1.30 on Xubuntu 16.04 and at work, Geany 1.31 on CentOS 7 (under Xfce on both). However, they look really different and I suspect it's because of GTK2 vs. GTK3, but that's where I'm not 100% sure. It's purely my personal preference but I vastly prefer the GTK2 look and feel to GTK3.
As Matthew said, the themes are different. Not all distros make much effort to match GTK-2 and GTK-3 themes. Linux Mint for example tries very hard to make both GTK2 and GTK3 versions of their standard themes, but many distros don't. But of course there is little or no effort to match between distros. All you ca do it try to find themes that are similar enough that you can live with.
Good Luck!
Cheers Lex