How to interpret 主题先行？
主题先行, as you said, it literally means "Themes go first".

But, "70年代" is the Cultural Revolution period, I suppose it refers to the situation that before artistic creation, the central theme has been decided by political factors; people has to do the creation accord with that.


  “主题先行”指十年动乱期间，根据政治需要，先确定所谓的“主题”，然后再根据“主题”的要求填进“人物”和“情节”，使“主题”得以表现的一种文学创作方法。