Well all the games you are running will be at a lower resolution than your monitor, which is fine, and Launchbox itself can happily run at that resolution also (although the UI will look rather large) but this theme specifically is coded for 1080p, well i say coded, its more likely that he is simply using 1080p art assets and placement, so they dont scale correctly for you
Most users these days have at least 1080p displays, even for emulation purposes, so most themes are designed with 1080p widescreen in mind. Some themes with the way they are coded and what art assets they use dont care what the monitor resolution is, but others with specifically created and placed artwork will.