I'm using elementary OS Loki 0.4 with the Arc OSX themes for GTK+ but the UI in apps is very ugly, can someone help me?
You have installed a theme which generally only applies to applications which use GTK+ 3, for example all default apps in elementary.
There are other toolkits used by developers for Linux apps where GTK+ themes will not be applied at all or at least differently. Here is a (non comprehensive) list of toolkits and notable examples of apps using them:
- XUL is used by Firefox and Thunderbird
- Qt is by used by VLC and apps from the KDE project
- wxWidgets is used by FileZilla
- VCL is used by LibreOffice
- GTK+ 2 is generally used by older apps and its themes are not compatible with GTK+ 3 themes
QGtkStyle (link for general info, might not apply to elementary) imitates the look of GTK+ 2 themes in Qt apps.
I'm not really sure, but I believe the way the FileZilla version available in Loki is compiled, it also tries to imitate the given GTK+ 2 theme. So in order for the app to look better, you have to find an appropriate GTK+ 2 theme and install it as well.
According to the README.md of the arc theme you need to also install the gtk2-engines-murrine package.
For the theme to function properly, install the following GNOME Shell, GTK 3.14 - 3.22 The gnome-themes-standard package The murrine engine. This has different names depending on your distro. gtk-engine-murrine (Arch Linux) gtk2-engines-murrine (Debian, Ubuntu, elementary OS) gtk-murrine-engine (Fedora) gtk2-engine-murrine (openSUSE) gtk-engines-murrine (Gentoo)