Whenever I interact with a bootstrap input on the web browser, I can right click, select "Inspect Element ...", and view the CSS properties under "Rules" or "Styles" on the right side of the screen. One particular style rule stands out:
.form-control:focus {
border-color: #66afe9;
outline: 0;
-webkit-box-shadow: inset 0 1px 1px rgba(0,0,0,.075),0 0 8px rgba(102,175,233,.6);
box-shadow: inset 0 1px 1px rgba(0,0,0,.075),0 0 8px rgba(102,175,233,.6);
}
The key to the blue color of the input lies in the values of the colors (both the hex #66afe9 and the rgba(102,175,233,.6)) within this code snippet. To change the input's color, you would need to adjust these values. However, it's worth noting that bootstrap may use variables for preprocessing the CSS, so modifying a variable value could be another way to achieve the desired visual effect.