Your site doesn’t look so bad to me on mobile. This image is 2/3 scale:
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There is definite room for improvement, but while things could be smaller, they don’t seem particularly oversized to me. Don’t forget a lot of people do not see so well, so large type may be appreciated!
A media query on a CSS stylesheet is basically like this:
@media screen and (max-width: 767px) {
div.recent-posts2 {
width: 99%;
}
/*more rules as needed*/
}
The rules within will only apply to screen widths no more than 767px wide in this example. The mobile emulator that is part of Chrome developer tools works pretty nicely. I believe all major browsers have something similar. Learn your way around the tool of your choice. It makes identifying which styles apply to what very easy and lets you experiment with different styles and selectors. Your changes in the tool will not persist. You can reset to the original styles by reloading.
In Chrome developer tools, the mobile emulator is toggled on and off by the overlapping rectangles icon at upper left. The square with arrow icon turns on the element selector tool. Pick the element on your page you want to inspect. On the main window you can pick different devices to emulate and toggle between portrait and landscape. The blue bar at top shows your theme’s media query breakpoints. You should try to use the same breakpoints for your media queries.
Once you decide which rules you want, copy them to where ever your theme lets you place custom CSS. This is often in the customizer. Create a media query there, then paste your desired styles inside.