Below is my CSS file. I want you to read it and understand it so I can ask you to make changes to it

body {
    font-family: Arial, sans-serif;
    margin: 0;
    padding: 0;
}

h1 {
    text-align: center;
}

button {
    background-color: #4285F4;
    padding: 8px 16px;
    color: #FFFFFF;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    font-size: 14px;
    margin: 0px 5px;
}

input[type="checkbox"] {
    cursor: pointer;
}

#resultMessage {
    text-align: center;
    margin-top: 20px;
}

#pageHeader {
    display: flex;
    align-items: center;
    justify-content: space-between;
    background-color: #f1f1f1;
    padding: 15px 20px;
    border-bottom: 1px solid #ddd;
}

#headerLeft, #headerCenter, #headerRight {
    flex: 1;
    display: flex;
    align-items: center;
}

#headerLeft a {
    font-size: 24px;
    font-weight: bold;
    text-decoration: none;
}

#headerCenter {
    justify-content: center;
}

#headerRight {
    justify-content: flex-end;
}

#homeLink {
    text-decoration: none;
    font-size: 24px;
    color: #333;
}

#homeLink:hover {
    text-decoration: underline;
}

#controls {
    text-align: center;
    margin-bottom: 20px;
}

#searchInput {
    width: 100%;
    max-width: 400px;
    padding: 8px 12px;
    font-size: 16px;
    border: 2px solid #000000;
    border-radius: 4px;
}

#searchInput:focus {
    outline: none !important;
    border: 3px solid #4285F4;
    padding: 7px 11px;
}

#settingsButton {
    background-color: transparent;
    border: none;
    padding: 0;
    cursor: pointer;
    margin-left: 10px;
}

#settingsButton:hover {
    color: #666;
}

#settingsButton .iconImage {
    width: 36px;
    height: 36px;
}

#mainContent {
    max-width: 1000px;
    margin: 20px auto;
    padding: 20px;
}

#tabsContent, #settingsContent {
    width: 100%;
}

.tabList {
    list-style-type: none;
    padding: 0;
    margin: 0;
}

.tabList li {
    display: flex;
    align-items: center;
    padding: 10px 15px;
    background-color: #FFFFFF;
    border-bottom: 1px solid #E0E0E0;
}

.tabList li:last-child {
    border-bottom: none;
}

.tabList a {
    text-decoration: none;
    color: #1A0DAB;
    font-size: 14px;
    flex-grow: 1;
}

.tabList a:hover {
    text-decoration: underline;
}

.tabCheckbox {
    margin-right: 10px;
}

.tabGroup {
    margin-bottom: 20px;
    border: 1px solid #E0E0E0;
    border-radius: 8px;
    overflow: hidden;
}

.groupHeaderContainer, .groupFooterContainer {
    display: flex;
    align-items: center;
    background-color: #f1f1f1;
    padding: 10px 15px;
}

.groupHeaderContainer {
    cursor: pointer;
}

.groupHeaderContainer .masterCheckbox, .groupFooterContainer .removeSelectedButton {
    margin-right: 10px;
}

.groupHeaderContainer .groupTitle {
    flex-grow: 1;
    margin: 0;
    padding: 0;
}

.groupHeaderContainer .groupTitle[contenteditable="true"] {
    outline: 2px solid #4285F4;
    background-color: #fff;
}

.groupHeaderContainer .editGroupButton:hover {
    background-color: #3071e8;
}

.groupHeaderContainer .removeGroupButton, .groupFooterContainer .removeSelectedButton, .removeTabButton {
    background-color: #d9534f;
}

.groupHeaderContainer .removeGroupButton:hover, .groupFooterContainer .removeSelectedButton:hover, .removeTabButton:hover {
    background-color: #c9302c;
}

.groupHeaderContainer .restoreGroupButton {
    background-color: #34A853;
}

.groupHeaderContainer .restoreGroupButton:hover {
    background-color: #2c8c47;
}