Yes. Very easy. Add this to user.css if no child theme or style.css if you're using a child theme:

.entry-content a, .entry-content a:link, .entry-content a:visited {
    color: #EE7600 (whatever color you want);
}

No problem!

Try:

#nav {
    margin: 0 auto;
    width: 990px;
}

Still fooling around with it.

What do you mean weight?

Create a child theme and if you mess up the PHP files, you can just delete the file from your child theme and it will go back to the defaul Arras files.

If you're not using a child theme place this code in your user.css, if you are using a child theme place this in your style.css file:

.search-results-content .searchform, .sidebar .searchform {
    width: 180px;
}

Yup. Exactly.

Try adding !important to each part that you're changing. For example:

.post { width: 1px !important; }

That would take a lot of work. Let me look into it for you.

You shouldn't edit the core files. It's better to just add the code I gave you in user.css.

Step 1: Open single.php

FIND

<h4 class="module-title">

REPLACE WITH

<h4 class="commentsnone-title">

Step 2: Add to user.css

.commentsnone-title { display: none !important; }

Step 3: Add to user.css

.nocomments { display: none; }