You're in the right section ... and vrkalak definitely knows his stuff when it comes to linux. Posted some interesting flux links in this section meself at one time. Was going through a fluxbox stage. Which still really like it ... Think flux is a versatile, super light and snappy DE. So these links might make fluxbox a lil clearer for ya.viewtopic.php?f=142&t=71571&p=418210&hilit=+fluxbox#p418210
Sure there's plenty of other flux related stuff in this section of the Magic Minty forums. Not sure about lmde ... Haven't had the pleasure yet myself. Should have the Mint menu and tools incorporated into it though .... right ? ( or wrong ).
Under the Mint menu look for summin related to log-in screen ... Whatever control center it has. Could set fluxbox to be the default there. For that matter once you've got fluxbox installed as vrkalak already laid out. All you should have to do is log-out ... It will/should take you to a log-in screen. Select yaself, then on the toolbar at the bottom of the screen a lil box should appear where you can select with DE ( desktop ) you want. Once you've chosen fluxbox there, enter your PW and hit enter. ( if you install any other DE's, you can also select and log-in to them that way too or switch between them in a couple secs.)
Should bring you into fluxbox. Fluxbox isn't really hard to figure out how to navigate around. Right clicking and so forth. Those links and others would show ya how to customize it to taste and set-up keyboard shortcuts etc.
after thought = Not sure if it's the controls for the login screen or the start-up manager. One or the other though. That's assuming the Mint menu is about the same in lmde or has similar control settings. Don't see why it wouldn't.