refine deprecated holder.js color syntax
refine deprecated holder.js color syntax

* refine deprecated holder.js color syntax
  * remove `#55595c:#373a3c`
* harmonize with all other holder.js examples

tagliala/bootstrap,yuyokk/bootstrap,kvlsrg/bootstrap,m5o/bootstrap,GerHobbelt/bootstrap,tagliala/bootstrap,stanwmusic/bootstrap,gijsbotje/bootstrap,gijsbotje/bootstrap,stanwmusic/bootstrap,peterblazejewicz/bootstrap,inway/bootstrap,yuyokk/bootstrap,bardiharborow/bootstrap,nice-fungal/bootstrap,peterblazejewicz/bootstrap,peterblazejewicz/bootstrap,coliff/bootstrap,inway/bootstrap,stanwmusic/bootstrap,twbs/bootstrap,tjkohli/bootstrap,fschumann1211/bootstrap,yuyokk/bootstrap,inway/bootstrap,kvlsrg/bootstrap,fschumann1211/bootstrap,bardiharborow/bootstrap,yuyokk/bootstrap,fschumann1211/bootstrap,coliff/bootstrap,tagliala/bootstrap,gijsbotje/bootstrap,m5o/bootstrap,fschumann1211/bootstrap,zalog/bootstrap,tjkohli/bootstrap,zalog/bootstrap,stanwmusic/bootstrap,kvlsrg/bootstrap,nice-fungal/bootstrap,twbs/bootstrap,GerHobbelt/bootstrap,GerHobbelt/bootstrap,zalog/bootstrap,bardiharborow/bootstrap,nice-fungal/bootstrap,bardiharborow/bootstrap,gijsbotje/bootstrap