Hi there!
The sidebar’s toggle in the upper right can be targeted with .sidebar-toggle and the actual panel that opens when you click on the toggle with .sidebar. Putting that together, you can change the background of both to black (#000) with the following custom CSS:
.sidebar-toggle, .sidebar {
background: #000;
}
The background and colour of the font in the comments section can then be changed with the following:
#comments #respond, .comment-respond {
background: #000;
color: #C5B358;
}
As an important note: You should not edit the theme’s files directly as any direct changes you make will be lost when it comes time to update. Instead, to add custom CSS, firstly set up a child theme or activate a custom CSS plugin. If you already have Jetpack installed then you can enable its custom CSS module.
In case you’re interested in how I found the correct CSS: I used the built in tools in my browser to inspect the theme’s current CSS and to live preview custom CSS.
If you’re relatively new to CSS and haven’t yet experimented with your browser’s built in tools then I thoroughly recommend taking a look through our guidance here. We have video walkthroughs for each of the most popular browsers:
https://en.support.wordpress.com/custom-design/how-to-find-your-themes-css/
Hope that’s helpful! I’ll be happy to help with any extra questions too.