In the mythology, beyond the fact that Ares is a great-nephew of Themis (since she is a sister of his grandparents Kronos [Cronus] and Rhea), the Titan-goddess never interacts with this god. And the Ares in this hymn is indeed so incongruous with the character we do encounter in the mythology as to seem an entirely different personage, as though he has already fully fleshed out into his later, statelier, more domesticated Roman counterpart Mars. For this reason Josho Brouwers of the Ancient World Magazine believes that "it seems unlikely that the poem is very early."
In the same way that, at Sparta, Aphrodite was worshipped as Areia, essentially a female version of Ares, here Ares seems to have popped out as a male version of Athena, this goddess typically being the one filling the role that Mars eventually does for the Roman state, as the deity presiding over the intellectual and strategic aspects of war rather than personifying the fury, disorder, ruin and horror accompanying it, as Ares typically does. Just as Athena was also, at Athens, closely enough associated with Ares to also be worshipped as Areia, "Of Ares," it shouldn't be such a stretch to see, in this hymn, the figure of something like an "Ares Atheneios."
Ares in the Sky
Regarding the words occurring right after the ovation of epithets which include "ally of Themis," the 1911 Encyclopædia Britannica "Ares" article says that:
It is to be noted, however, that in this little poem he is to some
extent confounded with the planet named after him (Ares, or Mars).
Interpreting this song as addressed to Areios, "Of Ares," i.e. the planet Mars, would make a lot of sense of it, if it is in fact fair to understand it as an astrological prayer in which the zodiacal ruler (who is borne across the firmament by "blazing steeds") is being invoked to grant favourable portents presaging peace rather than war. The light of the star is requested to "Shed down a kindly ray from above upon my life"—positive energy, literally.
The deity who personified the planet Mars was a star-god named Pyroeis, the "Fiery" One, so-called because of the reddish colour of his light. Theoi.com's "Pyroeis" encyclopedia entry follows the Encyclopædia Britannica's interpretation even more expressly, saying of Homeric Hymn 8 that:
Ares is here the star Mars.
The hymn is also the only source of an alternate parentage for Nike, the personification and goddess of victory, who here is Ares' daughter (whereas in every other instance her parents are the Titan Pallas and the River Styx, she herself being a Titaness, like Themis is). To me it seems quite in order to perceive her here as merely an abstraction, and the mention of paternity to be simply a reference to the idea that Ares, "War" (or rather the star Areios and all its symbolic significance?), gives rise to conquest, i.e. Nike.
The 2011 blog-post "Ares, Dike, and Themis", from the blog "Aspis of Ares: A Devotional Exploration of Ares, the God of War", and the 2017 article "Understanding Ares" on PaganBloggers: Of Magic and Devotion, in seeking to explain the connection of which you ask, both, incidentally, recall the myth which gives Athena her title of Areia.
The story goes that Ares, to either protect or avenge his daughter the Attikan princess Alkippe [Alcippe], killed Poseidon's son Halirrhothios [Halirrhothius], who had raped (or had attempted to rape) her. Poseidon filed a suit against Ares, whom the other Olympian gods judged in the first ever murder trial, upon the hill in Athens which afterwards would be called Areiopagos [Areopagus], "Ares' Crag." Ares was acquitted by the assembly, and the story is supposed to explain why at this court, from then on, whenever the jury's votes are equally divided, it is assumed that Athena, a silent member of the gathering, has voted in favour of the accused, thus acquitting him.
This seems to imply that the Olympians were themselves equally divided in the verdict against Ares until Athena, presumably the last one (of eleven members?) to vote, broke the deadlock by favouring Ares. And presumably also we are to understand this to have been the right decision. Later on in the same city, in Plato's time, the police of Athens are called "the officers of Ares" (Laws 670b).
PTH, the author of "Ares, Dike, and Themis", notes also the story about how the mortal king Sisyphos [Sisyphus] once tricked and kidnapped Thanatos, Death himself, who was then rescued from this ignominy by Ares. Based on this, Theoi calls Ares "the policeman of the gods", and as additional support for the notion, the site also displays an image from a Classical Attic Red Figure kantharos (again the region of Athens is in view here) depicting "The criminal Ixion who planned to seduce Hera" being "apprehended by the gods Ares and Hermes."
All these allusions seem quite tangential to me as far as attempting to elaborate on a link between Ares and Themis. The Titan-goddess's involvement herein, if we can call it such, is distant and inadvertent merely because "justice"—an especially broad and fluid term, especially over the millennia and into the modern era—is her domain. Maybe "Themis" in the hymn simply means "Custom" or "Tradition," which war has often been employed in order to maintain.