Child Theme and NAV Menu Widget

Hello, I have a question about child theme. Did made one with the Linea plugin. I'm always struggling with the homepage slider. Every time when I install an update the theme pictures of the theme, a wooden cabin and an orchid will wipe out my pictures. Do I need to backup some stylesheet in the child theme, to prevend this? And another question I have. Is it possible to get a bigger font size for the Navigation Menu Widget? The font size is really small, you'll need a magnifying glass to read... Thank you, Mare