Update brush-preset thumbnail: tone down background color

The new flat color #cecece doesn't let a lot of room in the value
to do nice contrast on the tool or on the stroke; I have to cap to
full white many things; it looks overexposed on a dark-theme
(Krita-default) while going only to #b4b4b4 ( not mid-grey, still
pretty light grey ) offers more possibilities. I'll keep the idea of
a flat background instead of my shaded square in my brushkit
but I'll just use #b4b4b4. (mnemonic: #before, before, before)