How are you going about applying them? If you're just hitting Apply Skin, that button will erase your previous skin and replace it with the new one. To mix and match different pieces of the skins that you want, you'll want to copy the code that goes with the skin and add it on top of the code from the other one. You can do this either by copying and pasting different snippets directly into the "Edit Profile" -> "Skin HTML" section, or by opening a document and adding all your code in there, before copying and pasting everything over. I prefer the latter, as it also lets me save previous versions of my profile. There may also be better ways to do this, but these are the method that I'm familiar with rn. (Sorry this is a lot of typing, but I hope it make sense)

You can definitely add your own boarders/fonts/images too! If you don't want to make something from scratch, you can read through the HTML for each skin and look for little sections that look like things that are easy to customize. A lot of links to tumblr or file garden are for images/gifs, and you can pretty easily grab the image links of gifs you upload somewhere, or find image links elsewhere on the internet, by right-clicking an image and clicking "Copy Image Address". Then you can just switch out the links. Colours are usually either RGB (rgb(###,###,###) or rgba(###,###,###,0.#), where the decimal is opacity), or hex code (######), both of which you can change by finding a colour selector online, a lot of which will tell you the RGB numbers or the hex code. This is something that you'd probably want to experiment in a separate document that saves previous copies of your code, because it's really easy to lose all your work if you don't know HTML, and you're just switching lines out. I recommend W3 Schools if you want basic examples of what different lines of code mean -> https://www.w3schools.com/htmL/default.asp