Hello,

You can change background color and logo size with few css code. About testimonial, slide in testimonials is working well you just need to touch content of testimonial and slide right or left it will work.

Please add below css in theme to change color and logo size

change height and width of logo as per your needed, if you decide to change testimonial bg color just change color code per your need in background-color

div#testimonial-content-section {
    background-color: #000;
}

img.custom-logo {
    height: 40px;
    width: 40px;
}

Follow my instruction to add above css Code

Go to Admin Panel >> Appearance >> Customizer >> Additional Css, Paste above code there and published then refresh you will see changes.

Hope this will helps for second and third issue

For first issue it will be more helpful if you share your site url with us. It will be more easy to further investigate.

Best regards

Hello @KONUR ALP

Add this code in Additional Css to hide ‘From The Blog’

.archive-posts-wrapper .section-heading-wrapper {
    display: none;
}

This css will help you to place site title next to logo in mobile display

@media screen and (max-width: 575px){
 .site-title{
	font-size: 24px;
 }

 .site-branding{
	display: block;
 }

 .site-identity{
    width: 47%;
    text-align: left;
    display: block !important;
    float: left;
    padding-left: 10px
 }

 img.custom-logo {
    max-width: -webkit-fill-available;
 }
 .custom-logo-link{
	width: 45%;
	float: left;
 }
 .wp-custom-logo {
	    display: block;
 }
}

Add all these Css code in Additional Css

Hope this helps

I see there you have blocked inspect option. Please next time enable inspect it will be easy for us to inspect actual class and help you little quick

Regards
Psink