SELECT 
  cscart_bm_layouts.*, 
  IF(
    cscart_vendor_styles.company_id = 2, 
    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 = 2 
WHERE 
  1 
  AND is_default = 1 
  AND theme_name = 'abt__unitheme2' 
  AND storefront_id = 1 
  AND (
    cscart_vendor_styles.company_id = 2 
    OR cscart_vendor_styles.company_id IS NULL
  )

Query time 0.00027

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_vendor_styles",
      "access_type": "system",
      "possible_keys": ["company_id_layout_id"],
      "rows": 0,
      "filtered": 0,
      "const_row_not_found": true
    },
    "table": {
      "table_name": "cscart_bm_layouts",
      "access_type": "ref",
      "possible_keys": ["is_default"],
      "key": "is_default",
      "key_length": "5",
      "used_key_parts": ["is_default", "storefront_id"],
      "ref": ["const", "const"],
      "rows": 3,
      "filtered": 100,
      "attached_condition": "cscart_bm_layouts.theme_name = 'abt__unitheme2'"
    }
  }
}