You can easilly make buttons on the start menu invisible using CSS, but you could also remove the start menu entirely and skip it if you'd personally prefer that.
To make buttons invisible, you would target them with a CSS Selector, and assign them display:none;
Here's a video on how to go about, when you don't know what you're doing, with playing with Monogatari to find CSS rules. https://www.youtube.com/watch?v=az6b9Qu7Qog Basically, you usually want to right click elements on the screen and choose "Inspect Element" to find information you can make into a CSS Query Selector. This will work for any website, not just monogatari.
For these buttons specifically on the main menu, you could select them like:
This one would select objects in the main-screen (which is the main menu front page) and then it would find buttons, and then it would only select buttons with the string "Load". You'd make a similar rule for each of the buttons, changing the text "Load" to a different one.
As for making the Start Screen not display at all and just skip it, you can find that in options.js
// Turn main menu on/off; Default: true *
Change this value to False.