I've been learning to code for some time and I've sure learned quite a lot, but styling has always been a pain in the ass for me (it's very embarrassing, I know). I absolutely have no idea how many pixels a nav bar should have or how wide any component should be. If it's just too small/big for me, I'll resize it. Size units? No idea. Don't get me wrong. Percentages, ems and metrics - I'm aware of its existence and what they do, but I do everything in pixels. I'm not saying that things I code look horrendous and not responsive. It's all try and error until things look aesthetically pleasing and it takes me so much time to make things look pretty.
Let's say I need to make something simple like a popup modal in CSS. I might wrap the hidden content in a div, with a class of modal, its content in a div with a class of modal__content, make a button to close it and some text in it. Semantically correct HTML is not a problem. Animations and colours? Fine, you got it. I just have no clue how to style it. That's when I realise I know CSS just in theory. I can code it, but I just have no idea if the modal should be 300x200 px box for a desktop screen or maybe use percentages?
If you have some saved articles or a book you could recommend, I'd be very very grateful.
I've used Bootstrap and Semantic UI. These are very cool and convenient tools, but I wish to know some in-depth guidelines when it comes sizing since I don't really know what something should actually look like. Most of the time I'm just eyeballing all font sizes, paddings and margins etc.
I'm not a good at graphic design either. Few people excel in both graphic design and in programming/scripting.
SitePoint has some books worth reading on the subject, the first especially (each available in paperback and as an ebook):
The Universal Principles of Design is likely to cover topics such as those you're asking about, although I haven't read it.
Graphic design relies a lot on convention (which is usually a good thing), fads (less so), and the preferences of the designer and client. However, there are various guidelines that are important to know. I'll mention a few here.
Large or bolder type, motion (videos and other animations), and contrasting borders and backgrounds draw the eye. Be careful not to give such visual emphasis to too many elements at the same time. (This is a common problem for home pages of organizations that many factions competing to highlight the thing most important to them.)
Adjacency implies relationships. For example, it's typically better to give headings, e.g. <h2>, a larger top margin than bottom margin.
Elements tend to need a comfortable amount of space between them without using too much screen space. When large spaces between content sections are desired, it's usually best to scale them down for mobile devices.
Animations serve multiple purposes. There are the various pulse/"throbber" animations to indicate that content is being loaded or some other kind of processing is taking place. There are transitions that show a non-instantaneous change between states, e.g. a menu opening/closing or change is views of a content/image slider. While others are mostly for aesthetic reasons, to add visual interest.
I'm glad you mentioned semantics. Do you use <label> elements where appropriate?
P.S. Too many people who call themselves "web designers" don't know, or refuse to acknowledge, the differences between graphic design for the Web and for print.