SELECT 
  cscart_bm_layouts.* 
FROM 
  cscart_bm_layouts 
WHERE 
  1 
  AND is_default = 1 
  AND theme_name = 'abt__unitheme2' 
  AND storefront_id = 1

Query time 0.00033

JSON explain

{
  "query_block": {
    "select_id": 1,
    "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'"
    }
  }
}