You are not alone! CSS is a pain. You'll learn by trial and error, and googling. here's some pointers:
The best way is to give it a class, like so
<input class='sheet-red' name="attr_str_base" type="number" />
What you name it doesn't matter, except it must start with sheet-. (Experienced roll20 users, before you complicate things, I'm keeping this simple.)
Then on your css tab, create a section named the same way
.sheet-red {
color: red;
}
This will set the color of that element to red. This means you can reuse the sheet-red class on as many elements as you like, and it will make them all red.
There is a complication: sometimes a style is already applied to an element, and you have to override it. You do that by making your code more specific: add extra identifying information, like saying "apply this style to inputs of the number type" You do that like this:
input[type="number"].sheet-red {
color: red;
}
You can include multiple styles in ab a class
input[type="number"].sheet-red {
color: red;
background-color: blue;
font-size: 12px;
}
and you can include multiple classes in an html element (separated by space)
<input class='sheet-red sheet-another-class sheet-yet-more-styles' name="attr_str_base" type="number" />