Is it possible to change the background color depending on which application is running currently?
My vim theme background differs with the terminal but I don't want to change the background of my terminal permanently just because of vim.
You can use multiple configuration files for alacritty. So instead of the default alacritty.yml you pass a different configuration to it.
alacritty --config-file ~/.config/alacritty/alacritty-vim.yml -e vim
This would spawn one instance of alacritty, running vim inside it, using a dedicated configuration file just for your vim preferences. This would allow you to give it a different appearance just for vim.
But it won't use that configuration on the fly if you start vim in a terminal. That does not seem possible currently, although I guess that such a feature could be added to alacritty's IPC msg mechanism.
vim myfile.js or vim myprojectdir
                
                For vim's colors there is vim's config, if that's what you are looking for. For example, I have
highlight Normal guibg=NONE
In ~/.config/nvim/init.vim, which makes vim's background transparent, i.e. matches it to alacritty's background. This works with transparency as well, if that's what you are after.