[Moving on while this is still in my head, the Class A uniforms.]
Next up, your Class A uniforms.
In real life there are a blizzard of variations that look different, but I'm going to presume only a few basic types per rank category of Enlisted (crewman recruit through PO1), Chiefs (CPO and up), Junior Officers (Ens-Lt), Mid-grade officers (Lt Cdr-Capt, or up to Fleet Captain if we use that rank), and Flag Officers (everybody above the mid-grade officers; admirals and commodores basically).
These I'm basically throwing most of Trek out the window stylistically, because Trek dress uniforms have generally sucked. There's just no two ways around it, they suck, especially from TNG forward - the FC and onward dress uniforms earn points for trying. but still don't pass. The moments you wear Class As are the moments outside of crisis where you most want to harken back to history, because history helps build esprit de corps and makes the solemn occasions that require dress uniforms more solemn. Trek dress uniforms rarely have tried for that. [Trek dress uniforms, even the best ones, have been "okay", especially for TV/cinema, but in need of improvement. When you aren't bound by the requirements of TV or cinema though...They're not really all that good, IMHO.]
It's no surprise what the base color of the Class A uniform should be, regardless of rank: White. Dress Whites are Naval. Dress Whites, frankly, look impressive (and blue as in the new US Army service dress or US Air Force uniforms, while it would also work, has historical and color associations to the Federal Blue of 19th century US Army uniforms, while White is more broadly naval). If not white (for instance if White is a dept color (if gray designates aerospace corps, what color is used for cadets?)), then Khaki, to harken back to the service khakis of World War II. [Aerospace Corps - fighters, bombers, combat transports, similar small craft that aren't shuttlecraft.]
Again, tunic and trousers arrangement. Because this is a space force, we should probably forgo buttons on the tunic, going for zippers instead. Just in case something shuts off the gravity during the ceremonies or something. Footwear should be dress shoes or dress boots, pick one for all personnel. If a species' feet really can't go into footwear comfortably (I can't think of any, but you never know), they can skip it, but again: What species would that even apply to? [I thought more on the question, and couldn't think of a Trek species that is bipedal where they didn't fit into humanoid footwear, but maybe they're out there. More broadly, I could see dress shoes on the Class As...But I think my preference would run towards boots. However, options are good.]
Shoulderboards of departmental color with rank insignia - sleeve stripes as classically used by navies for officers have some appeal, but are not as immediately readable as shoulder insignia, generally. Hence, we'll just go with shoulder insignia. *Perhaps* with a star on the shoulderboards, historically used to designate officers of the Unrestricted Line (AKA those eligible to be in command at sea), for those who are in one of the red, yellow, or gray departments (or at least not blue), who are in the departments which have the best claim to descent from the unrestricted line designation (science and medical/counseling officers would be staff corps officers, not line officers, in naval terms). Just to add an extra touch, although it could be left out if it'd make the shoulderboards look cluttered (or if the Marines use stars to designate general officers). [Yes, this RPG's universe uses Starfleet Marines. Otherwise, the star on the shoulderboards is a touch I wasn't too sure about, even without the Marine complication.]
Like on the Class Bs, enlisted insignia go on the shoulder sleeve, officers go on shoulderboards.
Not so many pockets here, it's a dress uniform and those don't generally need as many.
Nametags on the right breast of the tunic, not namestrips. Above can go personally-earned unit awards. [Namestrips are sewn or velcro'd on, Nametags are pinned on.]
Left breast has the commbadge. Wings/Warfare pins (if any) go above the commbadge. Ribbon rack (or ribbons and medals, or ribbons and miniature medals, depending on formality) goes below. Yes, ribbons, for all they're disfavored by sci-fi uniforms. In 20+ years of being a military geek, I've noticed something: It's possible, if you have the knowledge (and I admit I do not) to quickly read a servicemember's career in terms of achievements, training, deployments, etc. all from the ribbons on their chest. (I can't do that. I know a number of people who can.) This is an old and useful function of military uniforms...going all the way back to special togas worn by officers of the Roman Legions, if I recall right. [I'm a big fan of dress uniforms telling a story in part; ribbon bars do that expertly.]
One change I'm going to make purely out of personal taste: No neckties. None. Neckties are annoying and evil, says the guy who's never had the coordination to tie a knot, to say nothing of a tie. [Hey, if I'm designing, I get to abolish the necktie!]
Trousers should have departmental piping, or be straight white. Headgear should be combination cap for CPO and up through officers, or the classic sailor cap for enlisted, unless (say) your species can't wear headgear. [Headgear seems more necessary for a complete appearance with a dress uniform. It just does, I don't know why.]
If you want to go very traditional, include swords for Chiefs and officers outside the blue-colored departments. (The historical reason for not including the blues: They generally include chaplains, who traditionally are never armed, and they definitely include physicians, who shouldn't be using weapons. Everybody else traces their heritage to a combatant rating/career field.) That would probably be very controversial, but if it were to be included, it'd be something resurrected from the earliest days of Starfleet (or wet navies), so it need not mark any particular plot point. (On an OOC level, I mention swords because, lets face it, even in an age of phasers, Swords Make You Look Badass.
) [Swords...Yeah, I included this purely to think about. Does the modern USN (or their British/Commonwealth cousins) even use swords on the dress uniform?]
Multiple possible variations under the Class A category:
Mess Dress - for formal banquets, black-tie or white-tie equivalent. Miniature medals and ribbons for black-tie, full-sized medals and ribbons for white-tie.
Formal Dress - for military ceremonies, funerals, etc. Full medals and ribbons.
Service Dress - For office work at places like Starfleet Headquarters, wherever the Federation President's White House analog is (Paris?), etc., positions interacting with the public (recruiters, for example), and similar. "Where you're expected to wear a suit" is a good example for civilians.
Yes, that's complex. Again, I'm working and thinking from text, where these can be described in a paragraph per uniform on the webpage, if you make the uniforms tab deprecate (or whatever the term is) into something like the crew manifest tab does with a page per category (so one page for Class A, one for Class B, one for Class C). Drawing it may all be too complex, I realize. [It's complex, but definitely simplified from RL Class A uniforms.]
[I originally had a section on Class C (utility/working uniforms), but that's very
RPG-specific, and so I'm going to leave it out unless requested. Similar with the section on rank insignia, which boiled down to very technical musings, nothing useful. Instead, this will be where I stop. If you've read this far, you definitely have earned yourself a cookie.]