Rebel Alliance (Star Wars); Frontier Worlds/ Browncloaks (Firefly/Serenity); Early Federation (Star Trek); Tau Empire (Warhammer 40,000); Belters (The Expanse)
Fashion and culture in the Free Union is extremely varied, as it has been influenced by the various species that comprise it as well as whatever trends are prominent at the current time. There is a tendency in the Free Union to remain distinguishable from the austere culture of the Ascendancy, as it formed as a breakaway from that faction. Often this tendency expresses itself as deliberately embracing flamboyant, colourful and impractical attire, but resource scarcity and impracticality frequently makes these fashions out of the reach of the ordinary citizen. As the Free Union operates at the fringes of human space, it contains the greatest number of young colonies from Terra’s pre-cataclysm period, and the sleeper ships that seeded them actually held what is now the most complete archive of pre-Imperial human cultural history. Currently, the Free Union is enamoured by what cultural fragments remain from the 19th-21st centuries on Terra.
General everyday clothing has little uniformity, and even military uniforms can vary a lot between different groups. Although the styles seen in the Free Union are Wide-ranging, there is a general tendency towards loose, tough and “simple” clothing. Many tend towards a “frontier” look with breathable cotton layers, denim and oiled textile or leather coats. The wealthier fashionistas of the Union have currently embraced a retro trend that combines elements of Victorian and interwar garb with more modern and even futuristic elements (t-shirts and wearable tech being some examples, though not steampunk). Aliens in the Free Union are free to wear their own cultures' clothing or Terran fashions.
With its greater proportion of young colonies and alien worlds, Free Union tech is often jury-rigged from spare parts or repurposed from other devices, and has a cobbled together look. Panelling may be painted in different colours, and wiring might be visible (but if you are using electronics be sure that no components carrying a charge are exposed, for safety reasons). Resource scarcity could lead to bodged repair jobs, with components bolted together or even duct-taped - this sort of rough-and-ready look is very appropriate for more frontier-focused Free Union characters.
See this Pinterest board for an overall impression of the intended look and feel of the Free Union of Sovereign Worlds.