CSS,flexbox

Hello @keshavgoyal2807, I will answer the latter portion of your question first - You can learn advanced CSS libraries through a variety of resources.

Frontend Mentor has a well-made collection of these resource links on hand that you can try. Follow this link - (https://www.frontendmentor.io/resources) and choose as per your wish.

Regarding your first question, you can choose to take a trial run of Sketch for more pixel-perfect output. Alternatively, there is Figma, which is opensource and free, though how well each measure up to the task will be up to you as I have only superficially gone through them.

Hope this is helpful.

you did a great job, the best way by which you can estimate the correct dimension of a box is either by examining a image in photo shop or by buying the full pack course.