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

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_bm_layouts ref is_default is_default 5 const,const 1 Using where

Result

layout_id name is_default width layout_width min_width max_width theme_name style_id storefront_id
2 Main 1 16 fluid 280 1200 responsive Posh 1