Arch Linux

Gwenview doesn't require any specific icon theme, it will use your system wide theme

[k0ste@WorkStation ~]$ yaourt -R oxygen-icons
checking dependencies...

Packages (1) oxygen-icons-15.04.3-1

Total Removed Size: 35.15 MiB

:: Do you want to remove these packages? [Y/n] y
(1/1) removing oxygen-icons [----------------------------------------------------] 100%
[k0ste@WorkStation ~]$ gwenview
Could not find drkonqi at /usr/lib/drkonqi
Icon theme "oxygen" not found.
Icon theme "oxygen" not found.
Error: standard icon theme "oxygen" not found!
Invalid pixmap specified.
QTimeLine::setDuration: cannot set duration <= 0
Invalid pixmap specified.
QTimeLine::setDuration: cannot set duration <= 0
Invalid pixmap specified.
QTimeLine::setDuration: cannot set duration <= 0
Invalid pixmap specified.
QTimeLine::setDuration: cannot set duration <= 0
Invalid pixmap specified.
QTimeLine::setDuration: cannot set duration <= 0
mCurrentMainPageId == 'StartMainPageId'
[k0ste@WorkStation ~]$ yaourt -S oxygen-icons
resolving dependencies...
looking for conflicting packages...

Packages (1) oxygen-icons-15.04.3-1

Total Installed Size: 35.15 MiB

:: Proceed with installation? [Y/n] y
(1/1) checking keys in keyring [----------------------------------------------------] 100%
(1/1) checking package integrity [----------------------------------------------------] 100%
(1/1) loading package files [----------------------------------------------------] 100%
(1/1) checking for file conflicts [----------------------------------------------------] 100%
(1/1) checking available disk space [----------------------------------------------------] 100%
(1/1) installing oxygen-icons [----------------------------------------------------] 100%
[k0ste@WorkStation ~]$ gwenview
Could not find drkonqi at /usr/lib/drkonqi
mCurrentMainPageId == 'StartMainPageId

Add screens and logs.

What's the output of 'echo $XDG_CURRENT_DESKTOP'?

[k0ste@WorkStation ~]$ echo $XDG_CURRENT_DESKTOP
LXDE

Then change your icon theme with lxappearence to a theme that exists on your system. Qt5 apps will pick up the configured icon theme.

I try adwaita & nouveXT, gwenview still need oxygen.

Adwaita theme

Breeze theme

For me it picks up the theme I choose in lxappearance correctly. Not sure why it doesn't for you, but gwenview definitely doesn't require a specific icon theme.