Design style, informally (4)
I believe the answer is:
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">design style informally</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for deco that I've seen before include "Art's partner" , "Art -, C 20 style of ornamentation" , "Style of design popular in the 1920s and 1930s" .)