Hi,
Since the UI in most built-in apps has changed from EMUI 4.1 to 5.0, everything's different from what you might be used to from EMUI 4.1.
At the top of the screen in the Themes app, there should be 4 tabs. If you select the rightmost one, you should see a list of themes installed on your phone. Floating at the bottom should be a pill-shaped toolbar with a single icon labelled "customize" or something similar. Tap it. On the next screen, there should be one large and three small images, which allow you to customize different parts of the current theme. The largest, at the top left, should be "lock screen style". If you tap it, you end up on a list of thumbnails of lock screen styles. Somewhere in the list there should be the "magazine unlock" style. Hope that helps.
Of course, this only applies if the Themes app is behaving properly. I've run into all kinds of problems with the app, on several different Huawei phones, where all of a sudden some functions or UI parts went missing. Usually, that was because of me getting something wrong when flashing new firmware versions. Did you end up with a C900 version (EVA-L??C900B???) after flashing? Has it worked and just suddenly stopped working?