Add a theta angle inbetween axes and a line using Tikz

Question

I am a complete beginner to Tikz but this is what I have so far:

\begin {tikzpicture} [scale=3]
\draw [<->, thick] (0,2) node (yaxis) [above] {$i$}
|- (3,0) node (xaxis) [right] {$Y$};

\draw (0,0) coordinate (a_1) -- (2,1.8) coordinate (a_2);
\draw (0,1.5) coordinate (b_1) -- (2.5,0) coordinate (b_2);
    
\end{tikzpicture}

I would like a theta that corresponds to the angle between the upwards sloping line and the x-axis as shown below. (Ignore the other differences I am only interested in the theta element of the second picture). The arc around the theta also needs to be able fit if in the same manner if the slope of the line changes.

Answer 1

Here's a working example:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{angles,quotes}
\begin{document}

\begin{tikzpicture} [scale=3]
    
    \coordinate (a_1) at (0,0);
    \coordinate (a_2) at (2,1.8);
    \coordinate (b_1) at (0,1.5);
    \coordinate (b_2) at (2.5,0);
    
    \draw pic[draw,fill=blue!30,angle radius=1.2cm,"$\theta$" shift={(2mm,1mm)}] {angle=b_2--a_1--a_2};
    
    \draw (a_1) -- (a_2);
    \draw (b_1) -- (b_2);
    
    \draw [<->, thick] (0,2) node (yaxis) [above] {$i$}
    |- (3,0) node (xaxis) [right] {$Y$};
    
\end{tikzpicture}

\end{document}

Since the angle is a pic by itself, you may want to draw it before drawing axis and lines. It's better looking like that.
Another way to achieve this very easily is to use the tkz-euclide library.

The arc around the theta also needs to be able fit if in the same manner if the slope of the line changes.

I don't really understand what you mean by that, you may be more specific.