SELECT 
  cscart_bm_layouts.* 
FROM 
  cscart_bm_layouts 
WHERE 
  1 
  AND is_default = 1 
  AND theme_name = 'responsive' 
  AND storefront_id = 1

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.70"
    },
    "table": {
      "table_name": "cscart_bm_layouts",
      "access_type": "ref",
      "possible_keys": [
        "is_default"
      ],
      "key": "is_default",
      "used_key_parts": [
        "is_default",
        "storefront_id"
      ],
      "key_length": "5",
      "ref": [
        "const",
        "const"
      ],
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 0,
      "filtered": "33.33",
      "cost_info": {
        "read_cost": "0.50",
        "eval_cost": "0.07",
        "prefix_cost": "0.70",
        "data_read_per_join": "400"
      },
      "used_columns": [
        "layout_id",
        "name",
        "is_default",
        "width",
        "layout_width",
        "min_width",
        "max_width",
        "theme_name",
        "style_id",
        "storefront_id"
      ],
      "attached_condition": "(`betaatif_store`.`cscart_bm_layouts`.`theme_name` = 'responsive')"
    }
  }
}