Thanks to Tirifto for his sage advice (linguistically, stylistically, and generally); and for his website-generator, Pageling, with which I generate this site. Praised be!
The rose corner-ornament and divider ornament can be found at OpenVerse and Flickr, uploaded by CircaSassy in 2011. They apparently come from a specific book archived on the Internet Archive, “Desk book of type specimens, borders, ornaments, brass rules and cuts: catalogue of printing machinery and printers' supplies”. As the book is so old, these borders are under the public domain, barring the edits of CircaSassy; which are under the CC BY 2.0.
The twig-like corner-ornament is under the same license, from the same book, edited by the same user, and published in the same year. It can be found at OpenVerse and Flickr.
The floral frames— both the blue-hued and the rainbow— were authored by Jinterwas in 2010, and follow the CC BY 2.0. They are found at OpenVerse and Flickr.
By an unknown author, the floral divider for the front-page is available at OpenVerse and it's (seemingly) original source, RawPixel, which both label it as being under the CC0.
The drawing of Nagato Yuki used in the corner-ornament of the Post-list is from an unknown artist. The original source seems to be here, according to SauceNao (though the page is dead, now). If you find the author, do tell!