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

Query time 0.00017

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "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": 2,
          "filtered": 100,
          "attached_condition": "cscart_bm_layouts.theme_name = 'responsive'"
        }
      }
    ]
  }
}