I grepped now for that horrible background color #2a2c30 and found it in concrete/css/app.css

I tediously replaced several colors there and it seems to work.
My question is now: how can I make these style changes permanent (i.e. survive an update)?

Edit:
My changes to improve interface readability:
a.ccm-panel-add-block-draggable-block-type{display:block;height:90px;width:90px;line-height:0px;padding:2px;box-sizing:padding-box;text-align:center;background-color:#e9e3dd;border:1px solid #000;border-bottom:none;-webkit-border-radius:4px;-moz-border-radius:4px;border-radius:4px;-ms-border-radius:4px}
div#ccm-panel-add-block{background-color:#f5f3df}
a.ccm-panel-add-block-draggable-block-type span{position:absolute;left:0px;bottom:0px;width:100%;text-overflow:ellipsis;text-align:center;display:block;color:#000001;background-color:#fffffa;border:1px solid #000;border-top:none;font-size:12px;line-height:12px;padding-top:10px;padding-bottom:10px;overflow:hidden;white-space:nowrap;border-bottom-right-radius:4px;border-bottom-left-radius:4px}
div#ccm-panel-page,div#ccm-panel-compose-page{background-color:#f5f3df}
div#ccm-panel-page menu li a,div#ccm-panel-compose-page menu li a{color:#333c4e}
.ccm-ui .btn-info{color:#000;background-color:#5bc0de;border-color:#46b8da}
div#ccm-panel-add-block div.ccm-panel-header-accordion nav li>a{color:#10699d}

did you have a look at this add-on ?
https://www.concrete5.org/marketplace/addons/mrkdilkington-customize...

Ah, yes, I forgot to mention I indeed had a look at it, but unfortunately it couldn't do what I wanted. The customization affects the layout of the sidebar, but not its colors. The colors that can be customized with this add-on only affect the editing interface of the page content.