<main>
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox>
	<input type=checkbox>
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox checked>
	<input type=checkbox checked>
	<input type=checkbox checked> 
	<input type=checkbox checked> 
	<input type=checkbox> 
	<input type=checkbox>
	<input type=checkbox>
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox>
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox>
	<input type=checkbox>
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox> 
	<input type=checkbox>
	<input type=checkbox>
</main>

<main>
	<i>1</i>
	<i>2</i>
	<i>3</i>
	<i>4</i>
	<i>5</i>
	<i>6</i>
	<i>7</i>
	<i>8</i>
	<i>9</i>
	<i>10</i>
	<i>11</i>
	<i>12</i>
	<i>13</i>
	<i>14</i>
	<i>15</i>
	<i>16</i>
	<i>17</i>
	<i>18</i>
	<i>19</i>
	<i>20</i>
	<i>21</i>
	<i>22</i>
	<i>23</i>
	<i>24</i>
	<i>25</i>
	<i>26</i>
	<i>27</i>
	<i>28</i>
	<i>29</i>
	<i>30</i>
</main>

input {
	display: grid;
	width: 40px; height: 40px; margin: 0;
	appearance: none; -webkit-appearance: none; 
	cursor: pointer;
	background: #ddd;
	border-radius: 20px;
}

input:not(:nth-of-type(7n+1))::before,
input:nth-of-type(n+8)::after {
	content: '';	
	border-radius: 20px;
	pointer-events: none;
	grid-area: 1/1;
	z-index: -1;
}

input:not(:nth-of-type(7n+1))::before { transform: translatex(-60px); }

input:nth-of-type(n+8)::after { transform: translatey(-60px); }

input:checked { background: limegreen; }

/* a box's right borders */
input:not(:nth-of-type(7n)):checked + input:checked::before { 
	border-top-right-radius: 0; 
	border-bottom-right-radius: 0; 
	background: limegreen;
}
/* a box's bottom borders */
input:nth-last-of-type(n+8):checked + * + * + * + * + * + * + input:checked::after {
	border-bottom-right-radius: 0;
	border-bottom-left-radius: 0;
	background: limegreen;
}
/* a box's adjacent (rightside) box's left borders */
input:not(:nth-of-type(7n)):checked + input:checked + input::before { 	
	border-top-left-radius: 0; 
	border-bottom-left-radius: 0; 
	background: limegreen;
}
/* a box's adjacent (below) box's top borders */
input:not(:nth-of-type(7n)):checked + * + * + * + * + * + * +  input:checked + input::before { 
	border-top-left-radius: 0; 
	border-top-right-radius: 0; 
	background: limegreen;
}
/* a box's (in last column) left borders */
input:nth-of-type(7n-1):checked + input:checked {
	border-top-left-radius: 0;
	border-bottom-left-radius: 0;
}
/* a box's (in last column) adjacent (below) box's top borders */
input:nth-of-type(7n):checked + * + * + * + * + * + * + input:checked {
	border-top-left-radius: 0;
	border-top-right-radius: 0;
}

main {
	display: grid;
	grid-area: 1/1;
	grid:  repeat(5, 60px) / repeat(7, 60px);
	align-items: center; justify-items: center;
	margin: 0;
}

main:nth-of-type(2){
	pointer-events: none;
}

h4 {
	font-family: courier new;
	margin:0;
	margin-bottom: 10px;
	color:  darkslategray;
}
body {
	display: grid;
	margin-top: 25vh;
	align-items: center; justify-items: center;
	user-select: none; -webkit-user-select:  none;
	-webkit-tap-highlight-color: transparent;
}