body {
    font-family: Arial, sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    background-color: #f4f4f4;
}

.calculator {
    background-color: #fff;
    border-radius: 10px;
    width: 250px;
    padding: 10px;
    box-shadow: 0 4px 10px rgba(0, 0, 0, 0.2);
}

.display {
    width: 90%;
    height: 40px;
    margin-bottom: 10px;
    border: 2px solid #000;
    border-radius: 5px;
    text-align: right;
    font-size: 1.3em;
    padding: 5px 10px;
    background-color: #ffffff;
    color: #000;
}

.buttons {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
}

.btn {
    height: 50px;
    font-size: 1.2em;
    border: 2px solid #000;
    border-radius: 5px;
    background-color: #ffffff;
    color: #000;
    cursor: pointer;
    transition: background-color 0.2s ease;
}

.btn:active {
    background-color: #d3d3d3;
}

.theme-buttons {
    display: grid;
    grid-template-columns: repeat(2, 1fr);
    gap: 10px;
    margin-top: 10px;
    ;
}

#save-btn {
    background-color: #ffffff;
    color: #000;
    border: 2px solid #000;
}

#today-btn,
#light-theme-btn,
#dark-theme-btn,
#blue-theme-btn {
    height: 40px;
    font-size: 1em;
    border: 2px solid #000;
    border-radius: 5px;
    background-color: #ffffff;
    color: #000;
    cursor: pointer;
    transition: background-color 0.2s ease;
}

#save-btn:active {
    background-color: #d3d3d3;
}

#today-btn:active,
#light-theme-btn:active,
#dark-theme-btn:active,
#blue-theme-btn:active {
    background-color: #d3d3d3;
}