Very solid effort for your first attempt! There are a few tweaks and touches you should make to really elevate your craft, but this is a good starting point.
Breaking things down by criteria, spoilers from this point forward.
Implementation of Theme:
Sure, pretty clear here - Sergio is Lucas' light (and vice versa) and continues to be his inspiration even after his passing. The darkness is Lucas' depression from losing his husband and Gio's words, actions, and memories allow him to keep moving forward after his loss. Straightforward, can't really complain.
Story:
A few criticisms and then a positive that I want to highlight:
1) This one kinda stumbles out of the gate. It almost feels like you didn't know quite how to start, and so we get a kinda messy introduction to a bunch of characters at once. Besides a bit of initial confusion as to who's who, the initial party scene focuses way too much on the friend characters - Gio, the focal point of the story, barely even appears. If the story integrated Manny's and Ryan's thoughts and perspectives more consistently throughout, this could maybe work (tentative here because Gio really needs to be more present in this scene regardless), but they pretty much drop out after this in favor of more intimate (emotionally, not sexually. You know what I mean) interactions between Lucas and Gio.
2) While this VN is clearly purpose-built to be a tearjerker, it is possible to be too sentimental, and this one crosses into that space quite a few times. This story is really heavy-handed with the wistful nostalgia and sadness for the impending tragedy, and that's kind of hammered in by the looping "THIS IS A SAD STORY. YOU ARE SUPPOSED TO FEEL SAD" music. It might feel a little counterintuitive, but moments of levity and even mundanity can really help sell the more emotional moments - you don't have to be deeply sentimental the whole time. I think you were attempting this by opening with the party, but 1) you can sprinkle more of these moments in throughout; and 2) probably tweak the party scene - lean into the mood of that scene with the sounds and music. Instead of playing the dramatic music, have some tracks reminiscent of an action movie soundtrack, or other upbeat tunes you'd play at a hangout. You can sell the individual scene you're presenting rather than the overall plot.
3) We could also generally use more moments between Lucas and Gio that show us more about their relationship. Right now there's a lot of telling, especially on the bench, where the characters are essentially expositing their background and emotions, but it would be more effective if we could actually see these emotions in action rather than just hear about them. The positive I want to highlight is the poems. The first one especially really nailed the emotions of a high schooler writing a poem for their crush (so much so that it felt like there was a bit of personal experience there...) and the return poem with the callbacks to the original sold the characters better than anything else in the story. More moments like this, more glimmers of personality like the little "kill me" in the corner, all that goes a long way to making the sad moments really stick.
Presentation:
Decent - no issue with the stock sprites, but as mentioned the music was a bit repetitive and could have been used more effectively. Character sprites could have been introduced better and some of the poetry sections could have been split up into smaller chunks across more screens so they weren't taking up practically the entire screen.
Creativity:
So, "sad story about lover dying" on its own isn't necessarily new or original - that's a pretty basic but serviceable plot. I think what elevates the creativity to an extent here is the poetry, which allowed you to stretch your legs a bit more with the technique and style while also providing the story's best window into its characters. Definitely some good ideas here, just need a bit more fine tuning to make the whole project really "pop".
Stick with it! This was a good start and I hope to see more from you.