html {
    margin: 0px;
    padding: 0px;
    font-size: 150%;
    font-family: Arial, Helvetica, sans-serif;
}

body {
    margin: 0px;
    padding: 0px;
    background-color: lightgray;
}

body.level1 {
    background-color: navy;
}

body.level2 {
    background-color: green;
}

body.level3 {
    background-color: orchid;
}

body.level4 {
    background-color: orange;
}

#question {
    font-size: 2em;
    padding: 0.5em;
}

#content {
    background-color: rgba(255, 255, 255, 1);
    border-radius: 1em;
    text-align: center;
    padding: 1em;
    margin: 1em;
}

.choices {
    width: 100%;
}

.choice {
    background-color: lightblue;
    width: 80%;
    margin: 0px auto;
    margin-bottom: 0.5em;
    padding: 0.5em;
}

.choice.active:hover {
    background-color: lightyellow;
    cursor: pointer;
}

.choice.correct {
    border: .2em solid green;
    background-color: lightgreen;
}

.choice.incorrect {
    background-color: pink;
}

.hidden {
    visibility: hidden;
}

#nextBtn {
    font-size: inherit;
}

#scoreWrapper {
    text-align: right;
    font-family: monospace;
}

#problem.visible {
    animation-name: fadeIn;
    animation-duration: 1000ms;
}

@keyframes fadeIn {
    from {opacity: 0;}
    to {opacity: 1;}
}