Some themes may overwrite elements of other themes. I don't think there is a way to manually order what has priority in stylus currently.
Unless you mean you want to get rid of a theme, in that case just go to manage and delete it (stylus icon > manage > the cross next to what you want to delete)
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Unless you mean you want to get rid of a theme, in that case just go to manage and delete it (stylus icon > manage > the cross next to what you want to delete)