Added ethereum main net and sokol logos/variables

apps/block_scout_web/assets/css/theme/_ethereum_variables.scss

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+$logo-file-path: url("/../images/ethereum_logo.svg");
+
+$primary:       #12455b;
+$secondary:     #4786cb;
+$tertiary:      #77a4c5;

apps/block_scout_web/assets/css/theme/_sokol_variables.scss

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+$logo-file-path: url("/../images/sokol_logo.svg");
+
+$primary:       #539387;
+$secondary:     #77a4c5;
+$tertiary:      #38533d;

apps/block_scout_web/assets/static/images/ethereum_logo.svg

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+<svg id="Layer_1" data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 532.5 106"><defs><style>.cls-1{font-size:66.011px;font-family:Roboto-Thin, Roboto;letter-spacing:0.256em;}.cls-1,.cls-4{fill:#fff;}.cls-2{letter-spacing:0.247em;}.cls-3{letter-spacing:0.256em;}</style></defs><title>ethereum-logo</title><text class="cls-1" transform="translate(113.473 72.291)">ethe<tspan class="cls-2" x="191.141" y="0">r</tspan><tspan class="cls-3" x="229.506" y="0">eum</tspan></text><path class="cls-4" d="M57.548,17.661l21.7,35.992c-7.036,4.159-13.935,8.26-20.877,12.288a2.187,2.187,0,0,1-1.834-.109c-6.87-3.994-13.7-8.05-20.692-12.175C43.079,41.663,50.206,29.841,57.548,17.661Z"/><path class="cls-4" d="M78.125,58.923,57.231,88.339,36.322,58.906l.355-.441q2.551,1.51,5.1,3.019c4.457,2.627,8.953,5.19,13.35,7.913a3.386,3.386,0,0,0,4.129.044C64.672,66.1,70.188,62.93,75.67,59.7c.7-.413,1.433-.777,2.151-1.164Z"/></svg>

apps/block_scout_web/assets/static/images/sokol_logo.svg

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+<svg id="Layer_1" data-name="Layer 1" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 331.518 63.14"><defs><style>.cls-1{font-size:56px;font-family:Nunito-ExtraLight, Nunito;letter-spacing:0.256em;}.cls-1,.cls-3{fill:#fff;}.cls-2{letter-spacing:0.245em;}</style></defs><title>sokol_logo</title><text class="cls-1" transform="translate(135.687 47.74)">so<tspan class="cls-2" x="85.344" y="0">k</tspan><tspan x="125.271" y="0">ol</tspan></text><path class="cls-3" d="M128.7,31.492H40.539a1.825,1.825,0,0,0-1.824,1.825V57.57H82.931a1.216,1.216,0,0,1,1.18.922l4,16.016h50.424a1.825,1.825,0,0,0,1.77-2.267l-9.842-39.367A1.824,1.824,0,0,0,128.7,31.492ZM52.266,48.507l-2.384-9.538a.607.607,0,0,1,.589-.755h9.5a.606.606,0,0,1,.589.46l2.385,9.538a.607.607,0,0,1-.59.755h-9.5A.608.608,0,0,1,52.266,48.507Zm18.819,0L68.7,38.969a.607.607,0,0,1,.589-.755h9.5a.606.606,0,0,1,.589.46l2.385,9.538a.607.607,0,0,1-.59.755h-9.5A.609.609,0,0,1,71.085,48.507Zm31.587-10.293h20.254a.608.608,0,0,1,.59.46l5.073,20.292a.608.608,0,0,1-.59.755H107.745a.608.608,0,0,1-.59-.461l-5.073-20.291A.607.607,0,0,1,102.672,38.214Zm6.774,30.725-.771-3.085a.608.608,0,0,1,.59-.756h3.048a.608.608,0,0,1,.59.461l.771,3.085a.607.607,0,0,1-.59.755h-3.048A.608.608,0,0,1,109.446,68.939ZM126.868,65.1h3.048a.608.608,0,0,1,.59.461l.771,3.085a.607.607,0,0,1-.59.755h-3.048a.608.608,0,0,1-.59-.46l-.771-3.085A.608.608,0,0,1,126.868,65.1Z" transform="translate(-38.715 -23.604)"/><path class="cls-3" d="M38.715,65.636v7.047a1.825,1.825,0,0,0,1.824,1.825H80.722L78.5,65.636Z" transform="translate(-38.715 -23.604)"/></svg>