Help with themeing!

[Neuropsihopuppu]

So i want to start off by saying im a beginner but i have a few questions about modifying the webpage ,i am using it tools for personal use self hosted in a docker container and only available to myself on my private network ,i run a few other containers and have a dark/red color theme going on theme,i would like to change the color theme from grey and green to grey and shades of red ,and remove a few of the page elements like the "buy me a coffee" and links to github ,Twitter etc .

Am i allowed to do that for my personal use and if so can anybody guide me through it?

Thank you in advance and have a wonderful day