At this level almost nobody loses for lack of theory. Games are lost to the same old thing: a hanging piece in the middlegame, an advantage that slips in the endgame, a premature attack with your own king unsafe.
Studying more openings does not fix that. The opposite loop does: diagnose the exact pattern → train 5 minutes a day on your own positions → play → measure again. Improvement stops being a feeling and becomes a number going down.
Start with the free diagnosis: paste your username and within a minute you will know which pattern is costing you points, with your own moves cited as proof.