Here are the style rules for the indicated elements:
```css
/* 1. For the table element */
table {
border: 20px groove gray;
border-collapse: collapse;
}
/* 2. For the th and td elements */
th, td {
border: 1px solid gray;
padding: 5px;
vertical-align: top;
text-align: left;
}
/* 3. For the caption element */
caption {
caption-side: top;
font-size: 1.5em;
}
/* 4. For col element with the id firstCol */
col#firstCol {
background-color: yellow;
width: 150px;
}
/* 5. For col element with the id hourCols */
col#hourCols {
width: 75px;
}
/* 6. Change the background color of the thead element */
thead {
background-color: aqua;
}
```
These style rules can be added to the code6-1_table.css file to apply the styling to the corresponding elements in the HTML code.