SELECT 
  cscart_bm_layouts.*, 
  IF(
    cscart_vendor_styles.company_id = 137, 
    cscart_vendor_styles.style_id, 
    cscart_bm_layouts.style_id
  ) AS style_id 
FROM 
  cscart_bm_layouts 
  LEFT JOIN cscart_vendor_styles ON cscart_vendor_styles.layout_id = cscart_bm_layouts.layout_id 
  AND cscart_vendor_styles.company_id = 137 
WHERE 
  1 
  AND is_default = 1 
  AND theme_name = 'abt__unitheme2' 
  AND storefront_id = 1 
  AND (
    cscart_vendor_styles.company_id = 137 
    OR cscart_vendor_styles.company_id IS NULL
  )

Query time 0.00078

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.05"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_vendor_styles",
          "access_type": "system",
          "possible_keys": [
            "company_id_layout_id"
          ],
          "rows_examined_per_scan": 0,
          "rows_produced_per_join": 1,
          "filtered": "0.00",
          "const_row_not_found": true,
          "cost_info": {
            "read_cost": "0.00",
            "eval_cost": "0.10",
            "prefix_cost": "0.00",
            "data_read_per_join": "208"
          },
          "used_columns": [
            "company_id",
            "layout_id",
            "style_id"
          ]
        }
      },
      {
        "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": 3,
          "rows_produced_per_join": 0,
          "filtered": "10.00",
          "cost_info": {
            "read_cost": "0.75",
            "eval_cost": "0.03",
            "prefix_cost": "1.05",
            "data_read_per_join": "180"
          },
          "used_columns": [
            "layout_id",
            "name",
            "is_default",
            "width",
            "layout_width",
            "min_width",
            "max_width",
            "theme_name",
            "style_id",
            "storefront_id"
          ],
          "attached_condition": "(`s2cart`.`cscart_bm_layouts`.`theme_name` = 'abt__unitheme2')"
        }
      }
    ]
  }
}