Type /devtools in chat. Other -> Explore account data in Element Web (not "room account data", must be the global one), edit the m.widgets account data event to have the following content:

{ "stickerpicker": { "content": { "type": "m.stickerpicker", "url": "https://matrix.wah.su/stickerpicker/?theme=$theme", "name": "Stickerpicker", "creatorUserId": "@you:matrix.server.name", "data": {} }, "sender": "@you:matrix.server.name", "state_key": "stickerpicker", "type": "m.widget", "id": "stickerpicker" } }

If you do not yet have a m.widgets event, simply create it with that content. You can also use the client-server API directly instead of using Element Web.

The theme=$theme query parameter will make the widget conform to Element's theme automatically. You can also use light, dark or black instead of $theme to always use a specific theme.