.cell:nth-child(odd) { background:#CCC; }
.cell:nth-child(even) { background:#555; }
<body>
<div class="field">
<div class="cell">1</div>
<div class="cell">2</div>
<div class="cell">3</div>
<div class="cell">4</div>
<div class="cell">5</div>
<div class="cell">6</div>
<div class="cell">7</div>
<div class="cell">8</div>
<div class="cell">9</div>
<div class="cell">10</div>
<div class="cell">11</div>
<div class="cell">12</div>
<div class="cell">13</div>
<div class="cell">14</div>
<div class="cell">15</div>
<div class="cell">16</div>
<div class="cell">17</div>
<div class="cell">18</div>
<div class="cell">19</div>
<div class="cell">20</div>
<div class="cell">21</div>
<div class="cell">22</div>
<div class="cell">23</div>
<div class="cell">24</div>
<div class="cell">25</div>
</div>
</body>.cell:nth-child(2n), .cell:nth-child(n+5) {background:#CCC;}
.cell, .cell:nth-child(2n+6), .cell:nth-child(n+8) {background:#555;}
.cell:nth-child(2n+10), .cell:nth-child(n+12) {background:#CCC;}
.cell:nth-child(2n+14) {background:#555;}<style>
div{font-weight: bold}
div::before {
content: attr(data-1);
font-weight: normal;
}
div::after {
content: attr(data-2);
font-weight: normal;
}
</style><div data-1="Вот он" data-2="текст!">
выделенный
</div>
background: -webkit-linear-gradient(........); /*Safari 5.1-6*/
background: -o-linear-gradient(..........); /*Opera 11.1-12*/
background: -moz-linear-gradient(........................); /*Fx 3.6-15*/
background: linear-gradient(....................); /*Standard*/