Please use the Chrome web inspector to view the classes and CSS in use. You can then make these changes yourself. For the amount of questions your asking this is starting to become a customisation job and would require to work with a developer.
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1) You would apply the font CSS to the title class. So, to change the size:
.woocommerce div.product .product_title {
font-size: 12px;
}
Font CSS attributes: https://www.w3schools.com/css/css_font.asp
2) You can remove the border with CSS:
.summary-top .product-navigation .nav-previous,
.summary-top .product-navigation .nav-next {
border: none !important;
}
3)
.spb_accordion .spb_accordion_section>h4 a { {
padding-left: 0;
}
4)
.related-products span.price {
display: none !important;
}
5)
.archive span.price,
.archive .posted_in {
display: none !important;
}
6)
@media only screen and (max-width: 767px) {
.woo-aux-options-wrap.col-sm-12 {
display: none;
}
}
7)
.page-heading-breadcrumbs.fancy-heading .heading-text[data-textalign="center"] #breadcrumbs {
display: none;
}
8) There is no padding.
9) Have you set the max height in the Theme Options? You can always override it with a less value:
#mobile-logo.has-img img {
max-height: 80px !important;
}
10) You’ve not added the CSS, please add:
#overlay-menu nav>div {
margin-top: 100px;
}
11) You have a Mailchimp plugin installed which is causing the blue link color. In the plugin stop it from loading it’s CSS.
.