Hi,
<style>
#list-details1 .MuiDialogActions-root .MuiButtonBase-root {
background-color: #000000 !important;
color: #FFFFFF !important;
}
</style>
This is the code you need with the right selector.
It only deals with the text color but you know that to change the background color you just need to add background-color: #xxxxxx;
below or above the color:
line.
If you want to change the color of all the buttons of your block, then 
<style>
#list-details1 .MuiButtonBase-root {
background-color: #000000 !important;
color: #FFFFFF !important;
}
</style>
If you only want to change the color of the save button (which is the second button inside the modal for action button) 
<style>
#list-details1 .MuiDialogActions-root .MuiButtonBase-root:nth-child(2) {
background-color: #000000 !important;
color: #FFFFFF !important;
}
</style>
You only need to change list-details1 by the ID of your block where the modal comes from
If you only want to change the text color (and not the background of the button) just remove the line with background-color
in it
To change the color of the cancel button, on the left of the save button this should be this code:
<style>
#list-details1 .MuiDialogActions-root .MuiButtonBase-root:nth-child(1) {
background-color: #000000 !important;
color: #FFFFFF !important;
}
</style>