How to get chosen background-color on every line?

Please see the example here: https://www.w3.org/Style/Examples/007/evenodd.en.html

You would use CSS something like this

tr:nth-child(even) {background: #e5edf6}
tr:nth-child(odd) {background: white}

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Look at the markup your page is generating for the <tr> (Rows) that have only 1 date in

<tr class="rand"><td class="style35" height="25px"><strong>2017-05-01</strong></td></tr>

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There is only one column - you can style that until you are blue in the face it won't work. You have invalid markup which you should fix.
Here is the markup for a fully populated row

<tr class="rand"><td class="style35" height="25px"><strong>2017-05-12</strong></td>
<td><span class="style36">7</span></td>
<td><span class="style36">Peter</span></td>
<td><span class="style36">Avik</span></td>
<td><span class="style36">073-9832263</span></td>
<td><span class="style36"></span></td>
</tr>

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You should be using either <td></td> empty cells on the first example to pad out the cells without data
OR
Use colspan on a new cell to make it span the other cols.
As you have it your row (<tr>) does not span the table and will therefore not carry the colour across.
In your PHP you can do this
Option 1 (Pad the cells)

if (!$ret1)
{ $err = "QUERY FAIL: ".$SQL1. ' ERRNO: '. $mysqli->errno.' ERROR: '.$mysqli->error; trigger_error($err, E_USER_ERROR); }
while ($row = $ret1->fetch_array()) {

        $fornamn = utf8_encode($row["fornamn"]);
		$efternamn = utf8_encode ($row["efternamn"]);
		$medlemsnr = $row['medlemsnr'];
		$telnr = $row['telnr'];
		$mobilnr = $row['mobilnr'];
		$dag_bev1 = $row['dag_bev1'];
		$dag_bev2 = $row['dag_bev2'];
		$bryggplats = $row['bryggplats'];

echo "
<td><span class=\"style36\">$bryggplats</span></td>
<td><span class=\"style36\">$fornamn</td>
<td><span class=\"style36\">$efternamn</span></td>
<td><span class=\"style36\">$telnr</span></td>
<td><span class=\"style36\">$mobilnr</span></td>
</tr>";
}
else {
	// Pad out the row with empty cells.
	echo <<< ROW
		<td></td><td></td><td></td><td></td><td></td>
ROW;
}

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Option 2 (colspan)

if (!$ret1)
{ $err = "QUERY FAIL: ".$SQL1. ' ERRNO: '. $mysqli->errno.' ERROR: '.$mysqli->error; trigger_error($err, E_USER_ERROR); }
while ($row = $ret1->fetch_array()) {

        $fornamn = utf8_encode($row["fornamn"]);
		$efternamn = utf8_encode ($row["efternamn"]);
		$medlemsnr = $row['medlemsnr'];
		$telnr = $row['telnr'];
		$mobilnr = $row['mobilnr'];
		$dag_bev1 = $row['dag_bev1'];
		$dag_bev2 = $row['dag_bev2'];
		$bryggplats = $row['bryggplats'];

echo "
<td><span class=\"style36\">$bryggplats</span></td>
<td><span class=\"style36\">$fornamn</td>
<td><span class=\"style36\">$efternamn</span></td>
<td><span class=\"style36\">$telnr</span></td>
<td><span class=\"style36\">$mobilnr</span></td>
</tr>";
}
else {
 	// use colspan to fill out the row
	echo <<< ROW
		<td colspan="5"></td>
ROW;
}

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You might also want to take a look at PHP's HEREDOC syntax for your HTML output - it will make it a lot easier as it supports both single and double quotes in the string (without escaping) as well as variable evaluation.

Julian Hansen,
Thanks for your reply.
I try to understand the expression

echo <<< ROW
            <td colspan="5"></td>
ROW;

My knowledge is seemingly too little. Would you, please, explain it.

Read the link Ray posted.
HEREDOC is PHP syntax that allows you to have single quotes, double quotes and variables without escaping
the syntax is

$variables = "variables";

//left side of expression (echo, variable)
$output =  <<< LABEL
<div>You can put "anything" you like here you don't have to escape quotes (', ") 
and you can include {$variables} as well.</div>
<p>Makes for much neater code and less errors</p>
<p>The LABEL can be any string - the closing LABEL must be identical to the 
opening LABEL and must be flush against the margin. Both labels must have
 no trailing spaces - even after the ';' (in the case of the closing).</p>
<p>That is all there is to it</p>
LABEL; // No spaces here newline immediately after '; and LABEL flush against the margin
echo $output;

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Ray and Julian,
Thanks for your instructions and comments.

From Julian's first post I understand there must be five empty  cells added to the row should there not be anything appropriate for that day in the database. And I understand from your explanations of HEREDOC syntax the code starting with "<<< ROW..." does that.

When I apply the suggested code I get a 'syntax error code' message in my editor at the "else":

"<td><span class=\"style36\">$mobilnr</span></td>
</tr>";
}  
else... " <- this "else"

If you, please, take a look at that and suggest a code alteration, I'd appreciate it very much.

I tried to move the right hand bracket from
"error; trigger_error($err, E_USER_ERROR);  }"

and placed it just before the "else", it fills the row OK, but it doesn't print the text after the date.
I don't know what to do.

That was caused because I put the else after the closing for the while instead of the if should be

if (!$ret1) {
  $err = "QUERY FAIL: ".$SQL1. ' ERRNO: '. $mysqli->errno.' ERROR: '.$mysqli->error; trigger_error($err, E_USER_ERROR); }
  while ($row = $ret1->fetch_array()) {
    $fornamn = utf8_encode($row["fornamn"]);
    $efternamn = utf8_encode ($row["efternamn"]);
    $medlemsnr = $row['medlemsnr'];
    $telnr = $row['telnr'];
    $mobilnr = $row['mobilnr'];
    $dag_bev1 = $row['dag_bev1'];
    $dag_bev2 = $row['dag_bev2'];
    $bryggplats = $row['bryggplats'];
    echo <<< HTML
      <td><span class="style36">{$bryggplats}</span></td>
      <td><span class="style36">{$fornamn}</td>
      <td><span class="style36">{$efternamn}</span></td>
      <td><span class="style36">{$telnr}</span></td>
      <td><span class="style36">{$mobilnr}</span></td>
      </tr>
HTML;
  }
}
else {
 	// use colspan to fill out the row
	echo <<< ROW
		<td colspan="5"></td>
ROW;

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Julian and Ray,
Thanks for your excellent help!
Lennart (lericson)

For anyone coming upon this question in the future, the correct answer about alternating row colors is to use CSS.  For some reason this was not selected as a part of the answer.
https://www.experts-exchange.com/questions/29016902/How-to-get-chosen-background-color-on-every-line.html?anchorAnswerId=42097678#a42097678