I've always imagined the SolParty members themselves - as opposed to SolSec or SolMil (Solomani Military) - as dressed in the ancient traditional Terran clothes for politicians and bureaucratcs, that is, a 5631AD variant on the customary suit-and-tie (black suit, white undershirt, black tie, large SolSymbol pin on the tie). Faceless, Impersonal, Professional Official-Looking.
SolMil, on the other hand, dresses in utilitarian uniforms, usually black and sleek (and yes, with silver shoulder-mounted rank insignias). Combat uniforms are (for TL13+ forces) battledresses; otherwise they are sturdier version of the basic uniform, with a black flack jacket worn over it. SolNavy uniforms are high-tech vacc suits, light, comfortable and durable, unto which an helmet could be attached quickly when required.
Its really the "overt" elements of SolSec (as opposed to covert agents and sleeper-monitors) that has that Jack-Booted Thugs feel, just like Black Globe Generator has described.