Hey Juan, nice job! Looks close to the design, it's responsive and the theme switcher works nicely 👏 I just have a few suggestions for you.
You're missing the hover states for the cards. The cards should change colors slightly and the cursor should be a pointer.
The Overview - Today heading should be a gray color in light mode
It would be nice to have the cards laid out in two columns for tablet screen widths. You can create two equal width columns like this:
grid-template-columns: repeat(2, 1fr);
In your HTML you have
<p>F O L L O W E R S</p>. Instead of putting it in all caps and with spaces between the letters in the HTML, it's better practice to style it that way in CSS.
You can keep all your CSS in one file with media queries for different screen widths instead of splitting them into separate files for desktop and mobile.