After my recent post on how I thought the problem was solved, I found out that I was wrong. The theme changed back to WordPress’ default theme in a few days. In desperation, I overwrote my default theme with my custom theme and changed the identifying remark in the stylesheet to “WordPress default”. So everytime it reverts to the default theme, it will display the exact theme as before. The theme still changes automatically, but it looks the same like nothing happened. Case closed!